3rd Brake Light Help

[longfoto]

I've got a 96 Legacy GT with some ongoing electrical issues. One of which is the fact that my 3rd brake light on the spoiler, almost appears to be non-functioning. Upon further inspection in a dark garage, I can see it is lighting up but very very very faint. Before I go replacing it, how can I tell if it's the light, or related to an overall electrical issue with the car?

I'm also noticing the instrument gauges are very faint at idle, but get brighter as I accelerate (same with all interior lights in general). I'm noticing the same thing with the headlights.

Could it be a short someplace, and how to I check for such a thing? My mechanic says it's not the alternator. So hmmmm.... where to start.

[subikid90]

Sounds like a ground issue. Take the brake light out and test the positive wire to a good ground source and see how bright the tester light is. This is a good place to start. You can do it for your headlights also.