On closer inspection of my dash I discovered nothing was plugged into the headlight switch assembly. Previous owner had wrapped it up into some big club and shoved it up into the dash. Clearly there's some melting and burn marks. Before I slap a new harness and switch, I just have a few questions.

My haynes book shows a Y-shaped wiring harness and the turn signal switch built into the instrument cluster switch housing (ICSH). My '90 vert is clearly different as the turn signals are not built into the ICSH. So my parts '89 GXL isn't gonna have the appropriate harness, but I can cannabalize the headlight switch still, right? Where can I purchase a new harness, and what can cause this type of thing? I'd hate to slap some new stuff in there and the problem just happen again.

 

The harness from the GXL should provide what you need.

"This kind of thing" is a result of Mazda design decisions, they ran a lot of power through the switchgear and melting/shorting is the inevitable result.