Thermo-Fan Project

look up the pulse width controller on this site.
all the construction details are there including stuffing the perfboard to give a variable speed fan to keep your sink cooler.
PWC is better than lowered voltage since the full 12v is applied to the motor in short bursts and prevents it stalling, which can happen with lowered voltage drive. See if you can find some info on preventing PWC interfering with the input signal.