Diesel300 is smarter than me. OP just stated "300SDL". Does that tell a year range? Also "Brushes where bed" might mean something.

My 84-85 300D have just mechanical relays in the "blower control box". Even my 1996 Voyager has mechanical switches and resistors. My 2002 T&C blower is transistor controlled, but only because the optional digital ATC. It similarly runs "always high-speed" when the transistor "resistor"-pack fails, though it can be turned off.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans