Apply power to the fans as comrpess ignite suggested. You can tap power from the glow plug relay if you're short of wire.

Then assuming MB didn't change things too much from the 1987 model...

Power for the fans come through relays K9 (high engine temp) and K10 (AC pressure switch). The fans get full voltage with high engine temp. Voltage goes through a 0.5ohm(?) resistor with AC pressure.

To check engine temp trigger operation, locate the 3-pin temp sensor at the engine end of the upper radiator hose. Remove the 2-pin connector and jump the contacts. The fans should go at full tilt. If not, suspect relay K9. I think relay K9 is in the right rear position in the relay box just aft of the fusebox.

To check the AC pressure trigger, disconnect the wires from the pressure switch with pigtails at the AC receiver/drier then jump the connectors (not the pigtails!). The fans should go at less tilt. If not, check for continuity through the resistor behind the left headlight. If that checks, suspect relay K10, it's the next one forward of K9.

Did you check the fuses? If you have no record of new fuses in the last 10 years, replace all of them!

Sixto
87 300D