2011 hyundai i10 brake lights us, bulbs ok, switch ok and fuse fine got 12v on live side of switch wire still u/s wired externally to rear l/h lamp and it works also 3rd light on .Is a relay involved?

Light bulbs require power and ground in order to work. The ground circuit for the left rear combination lamp is shared between the turn lamp, stop lamp and tail lamp. Because the tail light still works in that specific combination lamp, we know that the shared/common ground for that assembly is OK. The power circuit for both stop lamps (left and right) have the same origin; voltage flows from the STOP LP FUSE (15A) through the DBC RELAY (when the stop lamp switch is pressed) and then to each stop lamp via their respective circuit branches. Because the right stop lamp works, we know that the shared/common fuse, relay and switch are all OK. I'd suspect an open in the left stop lamp power supply branch and would recommending checking for voltage on the green & black wire at the left rear combination lamp assembly to confirm. Regarding the left lamp on the hatch, the wiring diagram indicates these are not stop lamps rather tail lamps only. I'd recommend installing a tail lamp bulb instead of a brake light bulb into that assembly as a first step. You can read more here: https://www.yourmechanic.com/article/how-to-replace-your-car-fuse-box-by-jessica-howe

well first is the fuse. (if all else works ok, not stated)
no shorted bulbs, they can short. like that. and blow fuse.
the free book is here, see TJ? 1999
see last line WIRING, click that?
id work brakes first, 10x more simple.
see page. ,8w-51-4 (pdf page 134)
http://www.jeep4x4center.com/knowledge-base/index.htm?utm_source=cj#service

see fuse 2? brakes
it feeds brake switch .
id bet the fuse is blown,
if blown remove 4 lamps , blow now, wirnng shorts rear. (hitch hacks)
if not blown check wire, white-tn (tan) for 12v when brakes are applied, if 0v move forward do switch, brake,
the point that has 12vdc on one side and 0v other is bad.
wire, connector or switch.
if voltage is found to be low, that is a short.

If you replaced the brake switch at the foot pedal and it still doesn't work... both lights. Then you have a broken ground wire... both lamps feed from the same ground. If only one was working then you could suspect a live wire to just that lamp.

Did you try the passenger side brake bulb in the driver side socket (maybe the new lamp is bad)??

If it works, then the problem is either a bad ground connection / wire at / between chassis and the shell of brake lamp socket, or bad 12 volt wire at / between the common passenger 12 volt brake light wire in the harness to the center contact on the brake lamp socket.

My suggestion is if the brake lamp wire is ONLY for brakes - not the directional too, simply add a new wire spliced into the passenger side brake light wire and extend it to the brake light wire closest to the new socket that you recently installed on the driver side. Disconnect the vehicle brake wire from the new socket and wire nut it off or tape it up (so it can't short out if it somehow becomes energized again) and splice this new wire to the new socket.

Test & you're done!

the stop lamps are simple
a fuse , a switch, wires, and lamps
if you step on the brake and the voltage at the lamps is not 12v.
then the switch is bad, if there is 12v on one side and not 12v on the output side of the stop switch.
its very simple , flashlight simple wiring,

Turn signal tail lights and BRAKE LIGHTS NOT WORKING

It can be faulty brake light switch causing the problem or its blown brake light bulb. First check and confirm brake light lamp is working and that's not the case.

For turn signal not working it can be faulty turn signal switch or its issue with wiring:----

==========

To trouble shoot brake and turn signal related problem, check the help links mentioned below:-----

Click the link:----
1996 Buick Regal Grand Sport Brake light Switch Location? http://schematicsdiagram.blogspot.in/2012/04/1996-buick-regal-gran-sport-brake-light.html

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No brake lights on Buick? http://schematicsdiagram.blogspot.in/2012/04/no-brake-lights-on-buick.html

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Brake lights not working on 1989 BUICK REGAL?

http://schematicsdiagram.blogspot.in/2012/04/brake-lights-not-working-on-1989-buick.html

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How to replace Brake light switch on Chevrolet Tahoe?

http://technoanswers.blogspot.in/2012/02/how-to-replace-brake-light-switch-on.html

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How to replace Parking, turn, and Brake Lights on car?

http://schematicsdiagram.blogspot.com/2011/06/how-to-replace-parking-turn-and-brake.html

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Upper brake lights work, but low brake light not working?

http://schematicsdiagram.blogspot.com/2011/12/upper-brake-lights-workbut-low.html

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1997 Cadillac Catera: How to replace Brake Light Switch?

http://howtobyme.blogspot.in/2012/06/1997-cadillac-catera-how-to-replace.html

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2003 Chevrolet S10 Tail light Fuse http://schematicsdiagram.blogspot.com/2012/11/2003-chevrolet-s10-tail-light-fuse.html

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Chevrolet Parking light and Tail lamp light Wiring diagram http://schematicsdiagram.blogspot.com/2012/11/chevrolet-parking-light-and-tail-lamp.html

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Car Tail Lights Not Working?

http://schematicsdiagram.blogspot.com/2011/12/car-tail-lights-not-working.html

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Turn Signal Not working http://technoanswers.blogspot.com/2012/11/turn-signal-not-working.html

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Right turn signal not working on Pontiac Car?

http://schematicsdiagram.blogspot.com/2011/12/right-turn-signal-not-working-on.html

--------
Only one side indicator light working on car?

http://schematicsdiagram.blogspot.com/2011/12/only-one-side-indicator-light-working.html

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Turn signals not working on 1987 Jeep Wagoneer? http://schematicsdiagram.blogspot.com/2011/12/turn-signals-not-working-on-1987-jeep.html

1996 Chrysler LHS brake light circuit wiring diagram?

http://technoanswers.blogspot.in/2012/02/1996-chrysler-lhs-brake-light-circuit.html

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How to replace rear Brake lights on 2004 Toyota RAV4? http://schematicsdiagram.blogspot.in/2012/02/how-to-replace-rear-brake-lights-on.html

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One brake light on driver side not working, but other lights work?

http://schematicsdiagram.blogspot.com/2011/12/one-brake-light-on-driver-side-not.html

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2002 Lincoln Town car Fuse Box http://schematicsdiagram.blogspot.com/2012/11/2002-lincoln-town-car-fuse-box.html 2007 KIA Rio: Brake Light Fuse Box Diagram?

http://repairhelpcenter.blogspot.in/2012/06/2007-kia-rio-brake-light-fuse-box.html

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Overhead Brake lights not working on 1999 GMC Sierra 4x4?

http://technoanswers.blogspot.in/2012/03/overhead-brake-lights-not-working-on.html

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2004 Ford Mustang: Brake light Fuse Location?

http://howtobyme.blogspot.in/2012/05/2004-ford-mustang-brake-light-fuse.html

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These will help.

Thanks.

Try this possibilities to locate the fault. First get your voltmeter or 12v test lamp out. Check the 30 amp Brake (#2) maxi fuse in the under hood junction box.Fuse should have 12v at both sides at all times.
If okay proceed.
Go to the brake light switch. The orange wire should have 12v at all times. The white wire should have 12v when stepping on the brake.
If okay, proceed.
Next thing is the brake lamp relay in the underhood fuse box. Swap the relay with one that looks just like it and recheck the brake light operation. If the relay is no good replace it.
If the relay is swapped and still not working, see if you can feel the relay clicking when someone else steps on the brake pedal.

batt is power from the maxi fuse 12v all times
sls is the white wire from the brake switch 12v with brake applied.
gnd is body ground 0v all times
mfs goes to the multi function switch 12v with relay activated
the circuit is simple—power from the brake light switch white wires turns on the relay which then sends power back to the multifunction switch for distribution to the rear lights.
if you jump terminals batt and mfs the brake lights should come on.
If you have no power at terminal sls with the brake applied then you have a wiring fault white wire between the brake light switch and the relay
Thanks. Keep updated for any more query. You can rate this solution and show your appreciation.

The brake fuse #1 is located inside the truck below the steering column. I am assuming you have verified there is 12v power at the fuse & thru the brake lamp switch (orange wire power in & white wire power out, when pedal depressed) at all times. After the brake lamp switch , power goes (white wire) to the Turn signal/hazard lamp switch & out (dark green wire-rt brake lamp & yellow left brake lamp)

start with easy thing first are you sure both bulbs are ok ? get a test lamp
or make one!
get a good 12v bulb attach wires to side of bulb one to the terminal on base of
bulb,tape together now you can fined volts
hand brake on out of gear?

turn key on go inside car above the brake

theres a switch pull the plug off
put one end your test lamp to earth (the car body) and the other wire put to the terminals on the plug you pulled off if you get your test bulb to light its the switch thats faulty

replace switch if you have power at the switch and still no lights make a short bit of wire take back the insulation a bit so bair wire is showing make it in to a u shaped and put in to plug above the
brake that you pulled off you should now have volts(power) in the boot useing your new test lamp
to fned it make good all earthing points its somewhere between the switch and the boot?
if you fine when brake is pressed you get more than one light on its a bad earth
if you have no power at the switch ts prob fuse box fault tthry wll known for bad solder joins
good luck ps dont foget to replace plug

are the rear lights combined? ie sidelamp and brake lamp in one bulb .if so, there is a possibility that the bulb is not sitting correctly in the holder , causing both the side & brake lamp fuses to blow . if they are seperate bulbs you probably have ashort in the sidelamp system somewhere but the brakes could be a switch problem [easy check, find switch ,disconnect wires short across using wire or paperclip switch on ignition and check lights ] if they work the switch is the problem , [check fuse first] if they don't