I have a 2000 Ford Excursion. I went to hook up a

You have lost your GROUND on the Left. Just add a Ground wire to the Left and your lights will work.

This happened to my Expedition, 3 minute fix.

Standard wiring colors are as follows:

4-Way

5-Way

6-Way

7-Way

Green

Yellow

Brown

White

Red

Blue

Purple

Right Turn & Brake

Left Turn & Brake

Tail Lights

Ground

Backup Lights

Electric Brakes

Auxiliary Power
Green- Right Turn and Brake
Yellow- Left Turn and Brake
Brown- Tail Lights
White- Ground
Red- Reverse (Back Up) Lights
Blue- Electric Brakes
Purple- Auxiliary Power

It sounds like your green and yello wires are shorted together somewhere along the length of the harness. I would check the wires on the trailer first. Easier yet, if you have another trailer to connec or access to another to use for testing, connect to it and see if the problem repeats; if it does the trouble is in the vehicle's wiring.

dead sort in system wires to rear also check trailer bug in at rear bad for corrosion
there is a spliter jointer at the rear that splits power to both tail lights

hows your trailer hitch wireing old school 4 prong?check that wireing.the trailer hitct wireing generally ends up being the problem.checkwhere thay are tied in.and see if they are gettn grounded out.

it sounds like either the wires controling the left brake light and turn signal are borken or loose on your truck. or there could be a problem with the ground for the trailer hookup. you should take a volt meter and then have someone put the brake on and test at the hook on the truck side to see if you are getting any voltage there and do the same thing with the turn signal. if you do not have any voltage there you need to trace the wires back up into the truck to find where you do have voltage . you may find a broken wire as you go. How many wires are there in your trailer hook up?

Check a few things...
- The bulbs
- Check for ground at brake lamp connectors
- Check your wiring at the connectors for power
- Check for another fuse blown
- Do not hook up the trailer until you fix the ground problem on the trailer
- On some models, there is a module to control the rear/trailer lighting system. There may be a problem in the module.

TRY GROUNDING THE TRAILER TO THE TRUCK ELSEWHERE. HAD THE SAME PROBLEM, SO I GROUNDED THE TRAILER AND IT WORKED. GOOD LUCK

Does it have trailer light plug. After market trailer plugs cause all kinds of problems and should be removed and replaced when they do. Also check the bulb in another socket to make absolutely sure it is bad. I have been fooled more than once by newer bulbs that break connection towards the bottom and you can't see it

I take it the blinkers and brake lights work ok when not hooked up to the trailer? If yes, then the wiring to the tow hitch is not hooked up correctly. How many wires to trailer pig tail connection? U can have up to 7 depending on what you tow. Check to make sure these wires are hooked up from the truck to tow plug correctly: 7 wired---
brown-Rt turn and brake light, yellow-backup, white-ground, blue-electric brake, green-tail running lights, black-battery charger, red-left turn signal and brake light. Let me know if u have something other than 7 wire, I'll get the wiring color and source info for u.

exactly what does it do? please explain what happens when you press the brake turn on the light than brake signals ect.....