So the lights no longer work on my trailer. Well, most of them. I hitched it up to someone else and they all worked fine. So I know it's my rig.

After a lot of work, I ended up replacing the whole wire and plug from the hitch receiver to the first wire connection. (About 6' up and to the left rear quarter panel.)
BTW, Toyota no longer makes this part! So I spliced wires and all. Still no luck. Has anyone else had this problem?

 

My plug is spliced into the wires going into the drivers side brake, turn signal and backup lights.

Check your grounds.

Do you have a multimeter? Check all wires that they are at ~12V. If its lower you may have a faulty wire, a crossed wire, or a bad ground.

Oh and check the grounds

 

X4 on the grounds. Also a 12 volt test light sure comes in handy in identifying which wire does what. I like to just poke through the insulation with a sharp test light probe while a person is on the brakes or running lights or turn signal to see whats what. Retape it all up if you're worried about the elements.

 

I had the same issue with my 99 4Runner when I bought it. I am stretching my memory but, I think the problem was in the little black box attached to the trailer wiring harness. It is supposed to reduce the amps from your car system to run the trailer lights. I can't remember what the part is called.

You can order a new trailer wiring harness from various after market companies. You don't have to buy one from Toyota. I think NAPA even had one. The whole setup is about $100 bones though. It's not cheap.

 

Why not just get the plug and play harness from etrailer for about $30??

If you did not replace the converter [black box], that is most likely the problem. If you really wanted to jusst go the splicing route, you should have just spliced in a new converter.

There are dozens of trailer wiring threads on here you can search...

 

1+. This is your answer. The Toyota box is under the quarter panel molding and seems to be prone to failure when it short circuits. Go to NAPA/Walmart/Autozone and get the $30 generic box.

You can't just splice your wires to the trailer because the box converts your stop/turrn/tail signal into the 4 way/4 prong "domestic" car wiring scheme that trailers use.

The most time consuming part of the job is tearing out all of that plastic molding and carpet

 

general electrical has my thread regarding this.

34.75 for Hopkins #43135 from Amazon.

 

I also switched out the stock black box that was fried with a plug and play replacement, $34.95. Works like a dream.