What would cause the dome lamp fuse to blow?

Old fuses, or too small a rating, or poor connections at the fuse may cause it to blow once, but not again. Dome lights run Hot, so, burnt insulation from the heat can cause a short in the dome light housing. Also a pinched wire.

I had to put heat shrink over the connections in my '80 Chevy's dome lite, as the insulation was black and about to go. Bending the repaired wires away from the bulb helps it from burning again, too. Be sure to close all doors and let lamp Cool before you remove it--they get Real Hot. Wipe it with alcohol after you reinsert it to keep finger oils from causing hot spots that will cause the bulb to crack and fail.