HI all.
I have a 2000 GMC and we repaired the 2 light connections on the multifunction switch on the steering column. Strangely enough, some time after that I hooked up my trailer and found that all lights except the brake lights work. There is no power to either of the leads at the plug at the hitch.
Does anyone know if there is an fuse somewhere or how the leads are powered ? I have all lights on the truck.
The only other issue is that the 4 way flashers are "wonky" in that you have to hold the switch slightlly to the left to keep them on.

Any help would be appreciated.

Goes through the third brake light fuse. I think it's in the underhood fuse block and is a 10 amp fuse marked CHMSL. May also be in fuse block on dash.

So if that fuse is good, then where does the rest of the pathway go?
Does it get routed up through the steering column multi-function switch.

Anyone have the wiring diagram that they could Email me?

Again appreciate all the help. I have the manual but we just moved and its one of the boxes somewhere in the house.

If it came with the towing package there's a fuse for it. I had a trailer turn signal that didn't work and it turns out there's fuses for all the trailer lights.

Anyone know which fuse?

The CHMSL circuit is not routed thru the multifunction switch.

Power comes into the stop lamp switch on the orange wire from the stop lamps fuse. With the brake padal depressed voltage is sent to the white wire at the brake lamp switch where it goes to the underhood fuse block. Then it is split, with one leg going through the vehical stop fuse and the other goes through the CHMSL fuse, this has 2 circuits, one goes to the 3rd brake light and the other goes to the trailer harnes, shoud be light blue wire.

I should have asked what type of trailer plug does the truck have? Is it the round plug or is it the flat plug? It makes a difference because if the truck came with a round plug from the factory then you have the heavy duty trailer wiring, if you or someone else added a flat plug then you have the light duty trailer wiring and the brake lights do go through the multifunction switch and your problem is most likely in that, as you said you had to play with the hazard switch to get them to work. My first 2 responses only apply to the heavy duty trailer wiring.

Vehicle has heavy duty wiring with the 7 pin factory connector.

Mike are you saying that the brake lights fro the HD wiring DO go through the MFSwitch?

All fuses test good so the fault is somewhere between the fuse and the plug at the hitch.

Guess I'll have to get a wiring diagram :(.

I have heard that the newer GM pickups have to have an adapter of some sort to make all the trailer lights work. :confused:

The old standard trailer wiring will not work on my Son's and my nephew's pickups.
If I recall correctly the trailer has no brake lights. :confused:

Vehicle has heavy duty wiring with the 7 pin factory connector.

Mike are you saying that the brake lights fro the HD wiring DO go through the MFSwitch?

All fuses test good so the fault is somewhere between the fuse and the plug at the hitch.

Guess I'll have to get a wiring diagram :(.

No, if you have the 7 pin round connector then you have the HD wiring and it goes through the CHMSL fuse. Remove the under hood fuse block and look under it, mice like to go there and make a nest out of the wires, maybe they chewed through on of the light blue wires.

I have heard that the newer GM pickups have to have an adapter of some sort to make all the trailer lights work. :confused:

The old standard trailer wiring will not work on my Son's and my nephew's pickups.
If I recall correctly the trailer has no brake lights. :confused:

you can get a plug in harness to hook up an aftermarket trailer brake. it plugs in next to the brake pedal under the dash. it used to come in the glovebox of every truck that came with the HD towing package, but somewhere along the line GM decided to save a buck or two per truck and sell it separately.

Not sure about the trailer brake wire but I know that the lights won't work with the standard trailer wiring.

OK back to basics. Anyone have the correct wiring diagram?

Might check around this web site.

http://www.etrailer.com/faq-wiring.aspx

I have run into some issues along the same lines with my cousins 2003 Chevy. The problem when he hits the brakes one the 3rd brake light illuminates. We have tracked the problem down to the brake harness connector on the truck. Somehow when a trailer is hooked up the trailer's brake lights work but still not the trucks. We think it may be because the connector may have been damaged when pulling up a steep hill since it hangs down so low. This may be something to look into on yours as well its just a thought.

Update.
Problem was the multi function switch. Both the 4 ways and trailer brake lights run through this switch. Circuit failed inside the switch. Bought a replacement on EBay for $94.00 vs. GM in Canada at $473.