Hello, 2000 4runner. The ol` blown fuse under the hood for the instrument lights and tail lights. Completely disconnected the trailer plug in. Chased wires, wiggled wires, still blows fuses whenever it feels like it. Good for days, then poof. Im wondering if the (relay?black box next to the fuses) under the hood that says tail lights could be faulty? When the fuse blows under the hood, the little black box still clicks when I turn on the tail lights. I have been fighting with this for 2 weeks and am just asking if anyone has more ideas? Thanks guys, Frank

A relay will not cause a short, if it is not working properly it just won't complete a circuit and turn something on.
Now if there is a short downstream from the relay, when it is energized and completes the circuit the fuse would blow.

When you say 'completely disconnected the trailer plug in'-I feel the hairs on the back of my neck stand up a bit...

I want to be sure what you are saying-you disconnected the black box hidden behind the interior panel in the rear? Or simply snipped the wires leading to the 4 prong connector at the back bumper?

Have you seen your instrument panel lights come on when you step on the brakes for example?

I'm leaning towards the defective black box-some call it the trailer light controller, and I believe you will have a lot of odd issues if that thing is just unhooked and not replaced with something like the aftermarket Hopkins unit.

Here's some more reading:

Beware the trailer light controller - YotaTech Forums

I am not that far from you sir and glad to help you talk this out a bit.

In the meantime try to work your way backwards to when this first started to occur. Any recent activity with the cigarette lighters, stereo, or other electrical accessories?

Any reason to suspect a loose ground has just occurred, body damage, etc...

Its going to be a bit tricky because I checked on the fuses and they were fine. Happened to me when I installed new radio. Wiring on the 4runner is like an electrical loop from the fuse terminal under the dash to your tail lights and back. With the help of my electronic buddy we traced wires back to a couple of fuse terminal that were electrically bonded to each other. That was the cause of my running lights not working. With battery off I spayed it with some electrical contact cleaner. No problems for a year now.

You can never say never, oh wait, I just did.

OK, so be careful, Relays CAN cause a short, its just that in 30 some years of electrical troubleshooting, I've maybe seen it twice. And the first one was questionable.

In this particular circuit if the relay was the culprit, it would be blowing the 120A main fuse, because it is powered on both sides by that fuse. It is activated by grounding the switch, which is why you hear it clicking. Once activated it sends power to the 10A tail lamp fuse then it is distributed from there.

The logical and easiest thing to do is disconnect connector 1E at the Driver's side FuseBlock, or Body ECU. This will take out the entire tail lamp circuits on both sides of the vehicle including the rear gate. It leaves the front marker lights and the instrument panel lighting intact.

The next step is to disconnect connector 1B at the same location. This takes the instrument panel out of the loop.

Do this and report back the results. I can solve this in 4 fuses or less!