My third stop light does not work. The one behind the back window. I have checked the bulb and it is fine. I see some wiring going into the trunk and it is spliced with some black 2"x2" modules. I don't know where to start to look for the possible problem. Any help will be greatly appreciated.

The car you mention did not come from the factory with a third light unless it is a European version. If not it was added later in life. The kits to install were not too bad to put in but the wiring was only as good as the installer. The wires should go from the hat shelf to the left or right light assembly and be soldered in place. Look in each housing for added wires and trace towards the third light. Hopefully you will find it.
Turbo herb