Ok. I didn't realize you had a relay and To compound my error, I was looking at the TC wiring diagram! I figured the PO had monkeyed with your wires. Well he did but I think he meant to use the purple wire correctly. JB has it right. A purple wire should be fused and hot all the time but the PO mounted the wire on the wrong side of the fuse!
I don't have a good frame of reference. I wouldn't think that some corrosion on the fuse block would cause enough draw to melt your wire. I certainly could be wrong, but I'm thinking there is a short somewhere. Maybe the relay?