Not cheap but installation looks easy -
https://www.amazon.com/Derale-16795-PWM-Fan-Controller/dp/B00N0N75TI
How about a simpler Volvo two-relay controller with low speed for AC (existing aux fan trigger) and high speed over a coolant setpoint?
Is your alternator up for an extra 20 amps while idling?
Sixto
98 E320s sedan and wagon
02 C320 wagon