|Originally posted by Bloozberry:|
I apologize for the crappy resolution on the schematic but I'll walk you through it:
The ground wire leaves the light bulb, and branches off at S313 to some other component, but the other branch carries on to connector C305 pin D which is a four pin connector near the driver's side door hinge under the dash. From there it carries on to another splice called S311 which is hidden in a wire bundle in the same general area. The branch you want to follow carries on to connector C201 pin H, which is an 8 pin connector, again in the same general area. From there, the black wire carries on to yet another splice (S212) behind the center of the dash, and finally grounds out at G201, which is an eyelet with a screw through it in the vertical metal member that connects the cross bar under the dash to the center console structure. The ground is behind the radio but may be able to be seen by looking up from the driver's foot well towards the radio... what a complicated ground path!
Having worked on the wiring under the dash, I can offer some insight regarding this. The ground wires from all the stuff in the dash and around the interior are all routed to this one area behind the dash (near the defroster vent). The ground wires come together to form a wire bundle of sorts. There are too many wires to splice all together, so they're grouped into 3 separate splices. Those 3 splices have wires interconnecting them, presumably to prevent ground-loop interference. And from all that mess, there is a wire running to the ground point at G201.
That said, instead of digging around behind the dash to find out what's up with the ground wire, you could just run a new ground of your own. You could splice a short piece of wire into the existing ground wire, and ground it to the frame right next to the brake light. You could probably even use one of the brake light mounting screws to secure it.