If the fuse keeps blowing, when does it blow? Does it blow when you turn your lights on? Does it blow with lights off? Does it blow when you just turn on your foglights? Does it blow as soon as you put the new fuse in without starting the car?
I would say to disconnect your fogs main power, and then see if it blows.
the multiplexor is about $100 new from majestic. it is not that hard to change out, just real tight to work with where it is located.
One thing at a time. find the problem for the fuse first. i still don't think that has anything to do with your interior lighting problem. next time you do a install, make sure you tie up all your loose ends. sounds like a rookie mistake. it will get fixed, so don't sweat it.