78 FWB Mirror Thermometer

I don't think it is.

I've seen fiber optic ones on that era in Sevilles. Dunno about yours though. One way to tell would be to look for a fiber optic cable looking for a home inside the door. IIRC the Seville may have borrowed light from the LH dash area and sent it out through the door bellows.

Some lit up, some didn't. At least I think, I have a mint condition thermometer mirror for a 1971-1978 Eldorado sitting in my basement and it doesn't have any kind of fiber optic connection at all.

They so, using a fiber optic cable like the ones the lamp monitors use. The cable runs from the thermometer housing and follows the wires that power the windows and such through into the passenger compartment, then up behind the dash, where it terminates at the bulb house for either the light controls or the wipers. There is a small metal retainer that holds the end of the fiber cable in place, so make sure you get the bulb socket from the doner car if you're retrofitting the option onto your car. Make sure the ends of the fiber cable are clean for best illumination.

Craig