Chances are that is the 7 volt regulator that the temperature gauge relies on. Is your temp gauge working when the engine is fully warmed? It very well could be related to your rear lights, and perhaps a bunch of other maladies in wiring. Tread carefully, electrical is no fun.
You can find a free download factory service manual, as we as a wiring diagram in the WIKI link below. Add your location to your profile to help others help you.
Joel in the Couve