A previous poster mentioned that pressure and temp. activate one or the other. I do not know where these would be on a 300D. I would follow the wiring from the aux. fans in front of the condensor. I suspect you'll end up in the vicinity of the receiver-dryer if R-12 equipped. Hopefully someone versed in the 300D will offer more exact advise.
I agree about the 100C issue. I recently had a large amt. of work done on a 126 to include new radiator, fan clutch, t-stat, expansion tank, cap, hoses; coolant. My temp. gauge still goes to 100C in stop-and-go traffic, but quickly drops back to about 87C once underway again. This occurs with or without A/C and irregardless of the season. Cold weather is non-existent where I live.
The 126 owners manual says this is normal. I gave up worrying about something for which there is nothing to worry about.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.