You can buy a Haynes manual at Autozone. There is wiring diagrams and other trouble shooting things in the book.
Try PAINLESSWIRING.COM
This website has a lot of wiring for many cars including '92 Eclipse. http://www.modifiedlife.com/1992-mitsubishi-eclipse-car-audio-wiring-diagram/
for a stereo? go to the12volt.com
I would Invest in a Haynes Manual.
go to __the12volt.com __aojedam
http://www.installdr.com/Harnesses/Mitsubishi-Wiring.pdf
Begin by removing the wiring harness from the back of your headlight. Remove the headlight assembly retaining bolts. The headlight assembly will come out.
http://www.the12volt.com/installbay/stereodetail/1541.html
I'm looking for that too, if you find it, re-post anwser here! ;)
You can find a wiring diagram for your 1988 Mitsubishi Gallant at most Mitsubishi dealerships. Mitsubishi service manuals will have the wiring diagram and can be found at most local libraries.
haynes or palmers service manuals at autozone,advance auto, or Orielly's
go to installdr.com to see how to remove and install a stereo out of any car