If (and that's a big IF) the above post is correct, and "Speed 1" output is the low speed you want from the M-B's system, and the fan speed controller is behind the glovebox and the resistors are on the right fender and the wiring diagram I'm looking at is correct for your car; then (and that's a qualified THEN) the wire you may want to splice the changeover relay into is a green/blue wire going from pin 8 on the controller to pin 5 on the resistor pack.

I'd double check all this with a meter, wait a few days to see if someone else chimes in, maybe start a specific thread "where are the blower resistors in 82 300TD" or "where is the blower controller" and maybe wait until Monday before splicing wires on your car. The Thanksgiving holiday weekend here may have taken some knowledgeable members off line for a few days.