1990 Chevy C30 Bus

You should just need to tap into the taillights. You'll need brakes, and left and right turn signal. It should only be a total of 4 wires: left, right, brake and a common ground for all.

The last trailer wiring harness I bought came with a wiring diagram.

~jp

Wrong. It's left, right, tail, and ground.

left=yellow right=green tail=brown ground=white (generally)

What you need to consider on this is the size of the wiring going to the back and how long it is. You may want to run larger wires from a point farther forward to prevent overload or voltage drop. The wiring on modern vehicles is mighty small sometimes. If you want to get real creative, you could run a fused 10ga from the battery, then trigger relays with the original wiring at the back. But that's another tutorial altogether.

Well, the wiring isn't the problem here, its the location of the wires on the bus. Where are they exactly? I've gotten underneath the back end of it and I don't see any wires leading back to the tail lights. I need a wiring diagram or a general indication as to where I can tap into those lines at. The following is a picture of the type of vehicle I'm working on.http://www.newbritainpolice.org/images/Post%2025%20Bus.JPG

Um...ok, yeah, I misspoke and meant taillights, not brakes... but I'm not wrong. Either way, we're talking 3 hots and a ground. Aside from that one thing, you basically typed the exact same thing I did, albeit with color codes.

Another thing, don't you think all that additional wiring you suggested is overkill? It's a 1990 C30. I seriously doubt the wiring is radically different than most other trucks from that era. It's probably 16-18 gauge wiring, and should be plenty big enough to handle the additional load.

~jp

Ok... That's a G30, not a C30. It's van-based, not truck based. No biggie, but that does change things slightly. You may have to access the wires from the inside of the van, not from underneath. I would look for an access panel of sorts on the inside.

~jp

The OP didn't mention what he's going to tow or what kind of lights it has on it. And using your example of 18ga wiring, then add say 3 198 HD bulbs, you'll fry that 18ga. Like I said, we don't have all the info, so we can't draw a conclusion as to whether his wiring will handle the load or not.