Trailer lights stop working when tow vehicle lights are turned on

Congrats, you have found Ford's super secret fuse that drove everyone mad for years!

Trailer lighting is notoriously unreliable, especially on commercially manufactured and wired trailers where the manufacturer tries to save money by using a chassis or body earth return instead of a fully wired system and the better quality lamps such a system would require.

In my experience eight out of ten trailer lighting problems are related to a ground or earthing fault or poor quality wire and poor quality terminals being used.
The other two out of ten related to problems with the plug and socket connection (car to trailer).

This is assuming the towing vehicle had the tow socket wired to a high standard in the first place and didn't use the notorious Scotchloc.
The Scotchloc device was adequate for some of the wiring to older vehicles that used thickly insulated thick wiring but they are totally unsuitable for wiring connections that are exposed to movement, dirt and moisture and the thin wiring of the modern vehicle. There are in addition to the blue coloured ones, also red and white more suited to thinner wiring and also a brown one for larger cables but blue seems to be the ones supplied most often and so these are the ones used most often and consequently are responsible for quite a lot of faults where a supply doesn't even reach the tow socket.

When searching for a fault it is best to check the towing vehicle electrics and the trailer separately. Plugging in a known good trailer or lighting board is the best way to check the car socket. I have made a box of tricks for checking both the car and trailer separately, including the grounds and supplies for fridge, battery charging, etc. It will also, with the aid of a lighting board, provide lighting to a towed vehicle that would otherwise have to be trailered.

The turn signal bulb circuit on the trailer MUST match the towing vehicles turn signals. Example: Tow vehicle turn signal light is the same bulb as the brake light, then the trailer must match. If the tow vehicle has a separate turn signal (amber bulb located away from the brake light) then an adapter is available that will send the correct power to the trailer so everything works correctly. The adapter can be found an auto parts stores, RV supply stores and some truck stops too.

you can add a receiver hitch made for that vehicle. for light towing you should be fine. it won't handle a car trailer or Large boat.

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Trailer Wiring Diagrams 4 Way Systems

4 way flat molded connectors allow basic hookup for three lighting functions; right turn signal / stop light (green), left turn signal / stop light (yellow), taillight / license / side marker (brown) and a ground (white).

4 way tow vehicle side.
4 way trailer side.
5 Way Systems

Same as 4 way system listed above but adds a extra blue wire for brake signal or auxiliary power. Used on pop up camper trailers or utility / boat trailers.
5 way tow vehicle side.
5 way trailer side.
6 Way System, Rectangle Plug 3/4 inch by 1 inch 6 way rectangle connectors right turn signal (green), left turn signal (yellow), taillight (brown), ground (white). The red and blue wire can be used for brake control or auxiliary. Use on a small motorcycle trailer, snowmobile trailer or utility trailer. Can also be used as custom wiring on trailers with 3 light/wire systems. See "Tow Vehicle Taillight Wiring" below.
6 way tow vehicle side.
6 way trailer side.
6 Way Systems, Round Plug

Round 1 1/4" diameter metal connector allows 1 or 2 additional wiring and lighting functions such as back up lights, auxiliary 12v power or electric brakes. Note: The black (sometimes red) 12v and blue electric brakes wire may need to be reversed to suit the trailer. Check with a test light or VOM. Uses: horse trailer, travel trailers, landscaping trailer, car trailer, etc.

6 way tow vehicle side.
6 way trailer side.
7 Way Systems

Round 2" diameter connector allows additional pin for auxiliary 12 volt power or backup lights. Uses: heavy duty landscaping trailer, car trailer, boat trailers, horse trailer, travel trailer, construction trailer, etc. Check with a test light or VOM.

7 way RV flat blade tow vehicle side.
7 way RV flat blade trailer side.

The fuses for the trailer tow adapter are in the normal fuse panels.

Engine Compartment fuse panel
101 - Is the battery charge circuit ( hot in run position )
20 - Left Stop / Turn fuse
19 - Right Stop / Turn
5 - Trailer tow reversing lamps & Trailer Tow parking lamps

Cab Fuse Panel
23 - fuse for the relay for the trailer tow battery charge circuit

All of these are in the owners manual

If you did not get an owners manual with your truck ( purchased used ), you can download one from Ford :
https://www.fleet.ford.com/maintenance/owners_manuals/default.asp

Most likely the brown wire from the trailer connector is connected to a constant battery voltage source, and it should be connected to the vehicle running light circuit, however someone might have wired the trailer connector up that way on purpose so that the trailer does not draw power from the vehicle running lights and possibly damage the vehicle running light circuit. (Dodge/Chrysler vehicles do have delicate lighting systems) Here are some images to help assist you, and the 4 way connector is the same as the 5 way connector just without the blue wire for the trailer brakes.

Review this information and links:
- Trailer 4 Way System Diagram
- Accesories and Instructions



Instructions
1.) Determine a suitable location for mounting the circuit protected converter in an out of the way spot near the left tail lights in the trunk or on the frame rail, if mounted under the towing vehicle.
2.) Ground the white ground wire by placing the ring terminal under an existing screw attached to a clean metal surface or by drilling another 3/32" hole and using one of the self tapping screws provided.
3.) Disconnect the tow vehicle's negative (-) battery cable.
4.) Cut the in-line fuse holder loop wiring.
5.) Using a ring terminal (3/8 inch for top terminal or 1/2 inch for side terminal), attach an in-line fuse holder (with fuse removed) to the positive (+) terminal of the battery.
6.) Route black 12 gauge wire from the fuse holder to the converter passing under or through the vehicle.
Note: When passing the wire through sheet metal always go through an existing grommet, add a grommet or use silicone rubber to insulate the wire from the hole.
7.) Attach 12 gauge wire to the fuse holder with the yellow **** connector.
8.) Reconnect the tow vehicle (-) negative battery cable. Note: See tow vehicle owners manual for any special battery reconnection and instructions.
9.) Determine if the towing vehicle has a 2 wire or 3 wire system.

2 Wire System

• Has same bulb for stop and turn signals.
• Some vehicles have a separate bulb for stoplights but also have a combination bulb for turn and stop. These cars should be wired as 2 wire systems, using the wires going to the common bulbs.
• Note: Attach the crimp on spade terminal provided to the red stop wire and ground it along with the white wire.

3 Wire System

• Has amber turn signals, separate.
• Has separate bulbs for stop and turn signals (both red).
• Note: Attach red wire to tow vehicles brake light circuit.

10.) Using a circuit tester or volt ohm meter (VOM), determine which wire attached to the left taillight assembly is the left turn wire. Attach the units yellow left turn wire to this wire using one of the blue wire taps provided. Determine which wire is the taillight circuit and attach the module's brown tail / license wire to it with a wire tap. Determine which wire is the stop circuit and attach the module's red stop wire to it with a wire tap (for 3 wire systems only, see step 9).
11.) Route the units green right turn wire to the right side of the vehicle. Determine which wire is the right turn circuit and attach the green wire to it with a wire tap.
12.) Install the fuse into fuse holder and test the installation with a circuit light or trailer.
13.) Secure all loose wiring with cable ties.

Testing Procedure
With the ground wire connected and all of the other circuits attached, attach the ground lead of a circuit tester to the exposed ground terminal of the 4-flat end. Activate the tow vehicle left turn, right turn, tail and stop lights one at a time. Probe the three receptacles of the 4-flat end to confirm proper functions. If testing with a trailer, make the proper connections and do the same test as the circuit tester using the trailer lights. If a function on the trailer lights does not work properly, disconnect the trailer lights, turn functions on vehicle off and recheck function with the unit with a circuit tester. Then check the trailer wiring for potential circuit problems.

I hope help you with this (remember rated this help).

Good luck.

Also try http://www.etrailer.com/faq-wiring.aspx. They have a great website with lots of info.

so, the trailer has no brake lights, or turn signals, just tail lights? Everything (all lights) on truck work ok? What do you have for trailer wiring hook up 4, 5, 6, or 7 way? There is no fuse, the lights all work off the tow vehicle lights, so if the tow vehicle lights all work properly, it's probably a wiring problem, such as bad grounds, loose connections, etc. Get a volt meter and check voltage at truck plug to see if power is getting that far. If truck plug in is ok, wiring problem is the trailer. Make sure all grounds are tight and rust free, that's a common problem.