Easy test for circuit integrity is to jumper the high pressure sw. at reciever/drier. If fan runs on low w/jumper, the circuit is fine , so I would suspect low freon charge [ not allowing high side pressure to get to fan cut-in spec .[ 20 bar on '86-87 124.030].
Gauges are best for testing the pressures , but on R12 systems [ yours], a peek at the sight glass should show no foam/bubbles if you have proper charge.
If fan does not run with sw jumper, than SB post of fuse, relay,resistor are next steps.......[ the load fuse for low fan is #7-16A on that chassis...]