The DOME fuse is labeled 20A RADIO in the engine bay fuse box in '88 and earlier MKIII's (and apparently some '89)... it's labeled 20A DOME in later models.
This diagram is for a '90 without the optional CD player. It's the closest diagram I've got a link handy for right now. The only real difference here is that the rear only has one pair of speakers on '88 and older.
http://www.cygnusx1.net/Supra/Librar...x?S=Main&P=134
Wire colors may vary.
If your dome light is out but your clock does remember the time when the car is turned off then the issue isn't the 20A dome/radio fuse. Pull the lamp housing apart and inspect the grounds for corrosion... they're renowned for it... then test the door switches.