The ground wires are on the back panel, (where the hatch latches). They are near the taillights, on the inside of the car. You need to remove a few push in clips, and then pull the cover back. There is a bolt with a wire going to it on each side. These are you grounds. I would carefully unbolt the wires, clean them with baking soda and a wire brush, then clean the spot they contact on the car.
I can't say this is the exact problem you are experiencing, but it is a common failure point.
The fuse box should have an a good explanation of what each fuse is, the taillights on your 04 might have the fuse under the dash. I can't remember.