I currently have no brake lights! I have checked the fuses with a digital multimeter and they are good. I have replaced the brake light switch. My cruse control works just fine. My turn signals work just fine. If I use the trailed brake controllers manual switch the third brake light comes on. What do I check next?

test your brake light switch

The fuse to check is 20A fuse #42 (brakes & horn) under the hood. Use a test light or multimeter to measure voltage to body ground at both test tabs at the top of the fuse.OR YOU CAN REPLACE THE SWITCH WHICH IS ABOVE THE BRAKE PEDAL,AUTOZONE,20 BUCKS,OR CHECK THAT TAILLIGHTS ARE GROUNDED AND NO CORROSION.

Replace the brake light switch then. It's on the top of the brake pedal.Also check fuse first and condition of brake lights.You should, ideally check the switch operation with a multimeter before replacing.

If the circuit has power and the fuse is good the problem is most likely the switch. Look under the dash above the brake pedal arm. There is a small plunger type switch which is activated when the brake pedal is depressed. Sometimes this switch gets loose and or out of adjustment. There are two nuts on the shaft (one on each side of the mount). They should be adjusted so the brake lights activate when the brake pedal is depressed about 1/2 inch. If adjustment does not activate the lights use a multimeter and see if the switch is good by touching each of the two wires and depressing and releasing the switch. If there is no difference replace the switch.

You need to find the source of the drain on the battery. To do this you will need a multimeter set up to read DC current (ammeter from zero to 1amp range). Switch engine off, switch radio off and disconnect the negative lead to the battery terminal. Connect negative lead of ammeter to negative battery lead and the positive lead of the ammeter to the negative battery terminal. With all electrical items switched off the current reading should be very low, no more than just a few milliamps to keep the dashboard clock running etc. If there is an appreciable current flow locate the fuse box and remove and replace each fuse in turn, note any changes in current and note down the fuse(s) that result in the greatest drop when the fuse is removed. Now switch on an interior light (open a door) just to show you how much extra current is drawn from the battery as a result of its coming on. Is this the sort of current being drawn from the battery when the door is closed? Check out that the circuit with the greatest current drain has a legitimate reason for being on (car alarm maybe). If there is no reason for the current draw then find the item left on when it should be off. A classic case is the trunk (boot) light or the glove box light being left on when the lid is closed. These lights are disconnected by contact switches that open (break the electrical circuit)) when when the lid is closed. Checking the trunk light is easy, just climb in and ask a friend to open and close the lid. The car doors too have contact switches by the hinges and these determine when the interior lights come on or off etc. Check all contact switches by pressing them open by hand and registering the current changes with their opening and closing.

The alarm system usually uses the parking lights, not the brake lights so you'll have to test the brake light system. If your car did use the Brake lights absolutely,,, then your brake lights would be good & that would generally mean it's the fuse or the brake light switch.

I suggest that you check the fuse first with a 12 volt test light or multimeter, if it is good with current coming through, you'll need to test the brake light switch,, it's located just above the brake pedal, with the brakes applied you should be getting voltage at both terminals (my chart shows those wires as black/white) if so, then it must be the bulbs.

there are a few other options but this is the basic brake light system.

good luck.

Did you check the brake light bulbs? Replace the bulbs if found faulty.

If the bulbs are not faulty, then the wiring may be damaged at some place.

Have someone press the brake pedal and you check the supply to the lights, with a multimeter.

Get yourself a cheap multimeter and a 12v test lamp. Start checking what has power and what does not. Follow your wires where you can. Check all the connectors. Use it to double check your fuse and your new switch.

Make sure the negative contacts on your brake lights have a good earth. The continuity or ohms meter on the multimeter is good for this.

Sounds like you have done a good job so far. When you have check all the possible problems check the impossible ones :)

If the brake lights aren't working, but the bulbs and fuse are good, then check for a defective brake light switch. It should be below the brake pedal. When the brake pedal is depressed it activates the switch, which turns on the brake lights. The switch can be tested with an ohm meter or multimeter. If defective, replace. Hope this helped and best wishes.

Check if you have electrical current at brake light switch.If not go back to checking the fuses and plugs.