The only sure way to find out is to put a bulb (21w/5w-12v) in and put a battery between either terminal and the ground (any part of the reflector). The brake light will light brighter.
"Wise men talk because they have something to say, fools because they have to say something." (Plato)
My eyes may be decieving me but I think you have a 1157 recepticle. One side is Batt voltage and the other would therefore be the ground side. Light bulbs aren't polarized so I dont think it would matter which is which.
Hard to explain, even harder to find anything in my quick online search. So I did the next best thing: took a picture. You'll have to work things backwards, but I think you should be able to figure it out from this. The 1157 is a dual contact bulb with a set of locating pins offset from one another. This feature makes sure you get the bulb in the right way when changing it. Each contact is one side of the filament and the other sides are tied to together and connected to the brass shell, which in turn is grounded. It may be hard to see the annotation in my photo, but the bulb is facing up with the locating pin closest to the base. The other pin is higher up on the shell. Hope this helps!
Like NM 59 said, the fixture looks like it is a two bulb/three filiment set up. the picture is upsidedown and cut-off for the upper turn indicator. The pictured lower two-filiment bulb mount (1157) has the smaller filiment for the running light, and the larger filiment for the stop/brake light. The top single-filiment bulb mount (1156) is specifically for the turn indicator. Now, if you want to wire it another way, then that's another story... :driving:
