I just want to check... I don't have a '90 so I'm not sure...

Explain the 'front temp. switch'. Are you sure it's not the ECT sensor which is wired into the ECM? That one has nothing to do with the fans.

When you jump the 'rear temp. switch' the fans run. Now have you run the car HOT and the fans don't run - even with a good fuse? Then that fan switch is bad. Remember, it's setpoint is hotter than the normal temperature, so the temperature gauge will go a little high before the fan comes on. Not all the way up into the red, tho...