It is located under the dash below ashtray. It will be 4 pin plug w/RED,Blue,White,Green wires.
If the 2004 GMC Envoy came with a tow package, then there should be a trailer brake controller harness. It would be located up under the dash near the brake pedal.
There are about 5 fuses for the trailer harness, Trailer Back up,Trailer left turn, Trailer right turn, Trailer Brake, and Trailer hazards. They are located in the underhood fuse box on the Drivers side of your motor. On my 2003 Silverado, the Trailer left/stop and Trailer right/stop fuses are located in the fuse panel just inside the driver's side door. (Not sure about the trailer brake or hazard fuses.....I wasn't having problems with those :). There is one additional fuse for the trailer harness. The 15A fuse for the trailer running lights is located in the fuse panel under the hood, next to the battery. The fuse is labeled "Trailer Park".
You need to purchase the add on harness from the dealer. This harness plugs into one of the junction blocks located under the driver's side of dash above your left foot area. The other end of the harness attaches to your trailer brake controler. The trailer brake "blue" wire in back is folded back and tucked in the rear harness on the left side of the rear frame rail. You may have to drop the spare tire to expose it and connect it to the trailer connector plug.
If there is a trailer wiring harness on the vehicle for towing unplug harness under drivers side rear tail light to eliminate the trailer wiring that's probably the issue then plug factory wires together
Check all fuses first, check the brake switch located under the dash above the brake pedal. If you have a hitch on the car check the trailer wiring harness.
It isn't there, Dodge didn't wire the Dakota with one. You will have to wire the brake controller your self.
Start with: Check fuses, brake light switch (at brake pedal), check for trailer harness problems, check bulbs.
There is a factory style plug available that either came with the truck or is available through the aftermarket. It plugs into the factory panel that is located on the firewall by the driver's left foot. The plug will have wires attached to it that are for the brake control. The only other thing that will be required is a 40 amp PAL fuse for the underhood power distribution panel. If you are using a factory harness it will have the fuse taped to it. If you are using an aftermarket one, you will have to supply your own fuse.The factory harness has 5 wires and are different colors than the brake control. One of these wires is for an illumination circuit that most brake controls do not have. The aftermarket harness should only have four wires and they are usually the same as most brake controls (red, white, black and blue). Just make your connections, plug it in and away you go!
Brake pedal switch, wiring, bulbs, bad trailer harness (if there), fuses...
Not all 1500's where built with it. If it is there it will be above the parking brake.
The wiring harness & plug for the brake control on a 2002 Ford Explorer (with the trailer tow pachage) is located on the passenger side of the dash just above the glove box. Open and fully extend the glove box door. The gray 6-way plug should be up and to the left. The brake control Ford adapter will plug-in directly and then can be wired to the brake control.
It may not have one. Trailer brake connections are not usually included in a tow package. Check with a GM dealer to find out. If equipped, it should be wadded up under the left side of the dash by the parking brake.