Fuse #9 blew on my 1991 300E. I tried to put a new fuse in there and it blew three more times. All this happened with the ignition off and doors closed. Somewhere there must be a short.

This is the 7.5 A fuse that is for the clock, radio, overhead light, sun visor light, and a few other things.

Any help and suggestion on how to proceed to check the faulty item? I understand that there might be hundreds of things that can cause this but any wisdom and experience are appreciated.