I HAVE JUST PURCHASED THIS CREAMPUFF, 1981 300 SD ... #12 FUSE IS GOOD, UNDERNEATH LOOKS NEW BUT NOTHING CONTROLLED BY IT OPERATES...ANY HELP????

 

I assume you've verified power to both sides of the fuse with a test light? If you have power on both sides then somewhere down stream a wire is broken. These fuses can look good but can have a hair line crack which renders them to an open circuit. They can also corrode at the ends, replace the fuse and report back.