Fuses often corrode and blow out all by themselves, so a blown fuse does not always mean a short.
Fuses don't cost much.
If I were you, I would just replace the fuse and see if it blew out again.
You could check the current drain and perhaps find a short by bridging the fuse contacts with a voltmeter. You could install a new fuse and then bridge the dome light contacts.
A corroded fuse is most likely, at least in my experiance, but anything on the circuit could cause a fuse to blow out.
Your seat belt warning buzzer could have a disconnected contact. A problem with the radio would be more likely if it has been replaced. The light ands its switch are fairly simple items and it is less likely that they could be bad. The door light switch could have become rusty due to humidity inside the door.
Look at a wiring diagram. Good luck!
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"