Voltage-dropping resistors develop a LOT of heat and they are a burn and/or fire hazard unless they're installed in the airflow from the fan. Just swap to a 12V blower motor- - - -it should last for a lot of years and they're not that expensive.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!