Aux fans are easy to test. For low speed, locate the AC high pressure switch on the drier. It's the pressure switch with pigtails. With the engine running, maybe AC in cooling mode as well to be sure, jump the pigtail contacts. The aux fan should come on at low speed. If not, check the resistor in the corner behind the headlight. For high speed, locate the 3-pin temp switch on the engine fitting that attaches to the upper radiator hose. With the engine running, jump the contacts on the 2-pin connector. The aux fan should come on at high speed. For further testing, there are separate relays for low and high speeds in the box aft of the fuse box.

Sixto
87 300D