I've been having issues with the tail gate glass defrost. Previously when you start the truck the rear defrost switch defaulted to 'On'. When I tried to turn it off it would blow the IGN/LMPS fuse that powers the gauges. Thinking it was a faulty defrost timer relay I ordered one from eBay: 'Grand Wagoneer "Jeep AMC Heated Rear Window 75-86 Relay NOS 5454614"
https://www.ebay.com/itm/284249137455
Here's the original installed:
But when I installed the 'new' one and it behaved the same - defaulted to on and blew the fuse when I tried to turn it off. Instead of tracking down the issue any further we bypassed the relay and now the switch on the dash is just a constant on/off switch. Problem solved. No more blown fuse.