rockman ~ Welcome to the Corvette Action Center Community ~
First off I'd like to welcome you to our community
We have a very strong L81 (1981) following here at CACC
Now, the defogger switch for the Rear Window... Well my thoughts here are that the original switch may have been changed because the original one had a melt down..
I say this because you mentioned someone increased the fuse size.. My hunch is there is a problem with the Rear Window defogger circuit, probably a short that draws way too much current. I would suggest looking at the strips in the back window and using an ohm meter checking for grounds at the window then from the switch back.
The Power Ant.. almost every aftermarket Corvette parts catalog has replacement parts, "Riks Unlimited Corvette Parts"
MidYear and C3 Parts is a pretty good C3 parts house, as is
Mid America Corvette Central and many others.
At the top of the page is a "Tab" called "PORTAL" if you go into the Portal you see many more places to check out with websites.
Welcome aboard and Enjoy the Ride!