The resistor is for low speed. If it is corroded like yours are, you probably have so little current/voltage going through the fan does not run.
Replace it, they aren't very expensive. If you want more fan for the AC, you can just jumper over it, and the fan will run at high speed instead.
The resistor is bypassed in the circuit containing the temp sensor, so the fan runs on high if the engine temp goes up to 212F.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!