This may sound too simple but I just went thru this over this past summer.
I go to start the 90GT, nothing totally dead. Look up and the dome light is on, turn key....nothing-dome light dims to nothing. Let key go-dome light comes back on. Turn on headlights, they work, hit the key-headlights go out-engine don't even try.
I put on jumper cables, condition remains the same- me
So I'm thinking starter selenoid right? Wrong!
I take the hot to the starter and put it on the battery and the engine cranks(If you have the mini starter, put the hot on the + battery terminal then touch the small wire to the + terminal)
O.K. now I'm really confused
Turns out that the short 4ga wire that goes from the battery to the selenoid had corroded right in the middle under the shielding. I never say that happen before, no tears or cracks in the shielding, but it had litterally corroded apart. I put a new ($5) terminal, wire, and connector on and it worked fine.
Check all the simple things like this, start eliminating switches, selenoids etc...... to make everything as basic as possible, then go from there.