Jon,

I think you have it figured out. One wire (for the small filament) is the taillights...these are wired in parallel.

The other wire is for the brake/turn lamp which has the large filament. If you have turn signals, then there is a separate wire for left and right. If you don't have a turn signal then they would be wired together the same as the taillights.

The ground path for all these items is through the lamp housing so it's important that all the bolting and connections are clean and tight. This includes the connection of the bed to the frame.

Stuart