1990stangGT

I have asked the guys over at stangnet forums but no one seems to be able to solve of even come close to helping me over there. Here is my issue I have a 99 Gt the heater core has been bypassed until I install a new one my father was bringing it to me and decided to but a small space heater in the ciagarette lighter to defrost the windows. As he drove 2 hours to bring it to me all of the sudden the turn signals,wipers,door locks,panel lights,dome lights,a/c blower motor blows out defrost but not vents,radio turns on but no sound mach 460 amps do not turn on,trunk button,and most important the diagnostics port will not power my xcal2 up but it works fine in my f-150? All of the sudden he said this happened so my thought was a fuse, I used a fuse check light on all interior and firewall panel fuses all fine there none blown. Then I thought possibly a ground or sort somewhere I checked the diagram in haynes and it is terrible I could point down to a few grounds these circuits ran to but not all? So beyond this I am pretty stumped as to what to do or where to check? If anyone could help me out or give me a better diagram I would love it because I do not want to take it to a shop and pay out the a$$. Thanks alot and any help would be useful.