Body Grounds - 1989 FSM:
Driver's Side Fenderwell, behind Electrical Fan Relay. Light Gage w/screw
Driver's Side rear of Cylinder Head - aluminum braid to firewall. Bolted.
Battery to Engine Block - near Dipstick attachment point. Bolted
Behind Driver's Side Cargo Area Linger - for rear lighting. Light gage w/screw
I believe there's one to be found near the battery as well, on the Passenger Side Fenderwell. I can't say for sure - but I seem to recall seeing one there, even though it's not in the FSM.
Second taking them apart - you aren't going to get a look under the lug and the sheetmetal without taking them apart. Take them apart, clean them with a wire brush, remove a small patch of paint around the screw, and apply Ox-Gard before reassembly. If the lug is too far gone - replace it. Might want to replace the screws as well, just to be safe. I'll usually put a washer under the screw head as well - it helps keep the lug from being malformed - and helps keep a nice even clamping load on the lug to the sheetmetal (apply a little Ox-Gard to that as well - can't hurt.)
I haven't found any grounds under the dashboard, but I'd have a look for one or more under there as well - the ECU ground is listed as going to the main engine block ground, but don't count on it. Same treatment there as with all the other grounds.
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