89 124 chassis should have a aux. fuse holder with a metal type fuse for the blower motor. Locate just forward of the fuse box.
The test procedure of the blower system is very basic. There is a three pin connector tucked behind and to the right of the brake booster. Very hard to reach. Remove the connector and test for voltage at the pins comming out of the firewall. You should have a pin that has 12v with key on. Another is a ground and the third is a control voltage that will range from <1v on low speed to about 6v on defrost. If these are in order then you need to access the blower motor and elctronic blower regulator. This is kind of deep so start there and report back.
Good Luck and Have Fun.