Originally Posted by markwb
That white lead in the pic looks as if it is cut off, dead ended. What'* up with that?
Okay, I think what we're looking at here is an aftermarket replacement 2-conductor socket (well, 3 if you count the ground connection on the side) for a dual-filament bulb. The brown conductor is the standard color for a GM running (tail) lamp exterior lighting circuit; the white one looks like generic color coding for what would be the brake lamp circuit, whose color varies from one GM model to another, depending, among other things, on whether the car uses the same circuit for both braking and turn signals, or whether like the Bonneville, it has a separate brake lamp circuit.
According to the manual, the backup lamp is supposed to be a #1141 bulb, which is a single-contact base like a #1156, not a dual-contact base like a #1157.
The brown wire runs out of sight off the edge of the photo, but I'm thinking it may lead to a spliced connection somewhere.
This is all guesswork on my part; maybe the original poster can get back to us here.