The battery to dip stick bracket, the motor to fire wall (near the MAP sensor) and behind the windshield washer bottle. Are the major ones, there are others hidden throughout the vehicle. The one next to the drivers side courtesy light, has caused me problems in the past.
Some of the grounds pass through the C101 connector (don´t remember if the 90 has one or not) and through the C100 connector, engine compartment side of the fuse box. I often test for voltage (lowest scale) from a ground wire to the chassis (using a tool to prick the wire/then reseal with a dab of gasket sealer), on both sides of a connector, whenever I have a questionable ground. There is sometimes standing voltage, like .4 or .8 volts on one side of the connector but not on the other side, a tip off, a ground or connector isn´t making good contact.
The C-100 connector (and the C 101 if present) are packed with some kind of di-electric grease or sealer, I´ve never felt comfortable, with just hooking them back up again and they are a pain (bordering on impossible) to get clean. So I unplug them with reluctance and usually just jump (solder) a wire around the connector.