Where is alternator fuse on 1990 Ford Thunderbird?

There isn't a fuse specifically for the alternator in a 1990 Ford Thunderbird. The alternator is directly connected to the battery and its charging function is regulated by the voltage regulator (which is either part of the alternator itself or built into the PCM - Powertrain Control Module on later models).

Problems with the charging system (like a failing alternator) won't typically manifest as a blown fuse, but rather as a lack of charging indicated by a low battery voltage. If you suspect an alternator issue, you should have the alternator and battery tested by a mechanic or auto parts store. They have the tools to diagnose the problem properly.