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I had an annoying and mysterious problem with my Volkswagen that I just solved last weekend. It's one of those things that my theories got very complicated because I hadn't checked the basics.
The symptoms were that the right brake light was significantly brighter than the left and when the brakes were pressed, the speedometer and the front fender lights came on dimly (like the running lights were on but at a low voltage). I tested where voltages were going, first with the bulbs in and then with the bulbs out. I got all kinds of wierd results, and I couldn't figure out anything conclusive.
The bulb for the tail/brake lights has dual filaments:
It's called a 5 and 21 watt bulb. The dimmer filament is for the tail lights, the brighter one is for the brake lights. The base of the bulb looks like this:
There are two studs at the base of the bulb, one for each filament, and the barrel is the common ground. The bulbs fit into a socket that has two contacts at the back, one for each stud:
I think the problem that existed was that one of the studs on the left bulb had become corroded, and so it wasn't making good contact with its contact. The former owner tried to get it to work, and in the process, got the bulb jammed into the socket sort of sideways so that the stud for the tail light was touching both of the contacts. That would mean that the tail lights would work as normal. When the brake light circuit was engergized, the tail light filament lit up (thus creating a dim brake light), and it also transferred some of the current to the running light circuit, lighting up the speedometer light and the fender lights.
The rule that I forgot was when you have an electrical problem, always make sure your connections are clean and tight. I didn't do that, and I ran around looking for wierd things in the wiring. Notice in the photo above of the base of the bulbs, the studs on the bulb on the left are all shiny, but the other bulb is only shiny where it rubs on the contacts. Well, I took some fine grit sandpaper, sanded off the studs on the (formerly non-working) left bulb, and put the bulbs in, and now they work just fine. Both brake lights are now significantly brighter than the tail lights, and the speedometer light and front fender lights don't light up when I push on the brakes.