Here's the background: I've got a Vintage Air Heritage unit in my truck, and the fan blows harder than I need or want, and makes too much noise (These are not uncommon complaints with VA). So I want to slow down the fan speed to less than the lowest of the 3 speeds VA provides. I asked the tech at VA if I could just add a resistor to either the power or the ground wires for the blower motor. He said yes "In theory", but he wouldn't recommend it. I couldn't get any reason why out of him, nor could I get an alternate recommendation. I was going to buy a big, ceramic resistor and give it a try, but became concerned about how hot it would get so I looked into PWM's and found one often used for trolling motors that had good reviews and ample power-handling capacity: https://www.amazon.com/gp/product/B07B4B2X35/ref=ppx_od_dt_b_asin_title_s04?ie=UTF8&psc=1 So I wired it into the low power wire to the fan, and it just passes the power through: No ability to decrease the rpm. The pot has no effect. In reading posts from other users there is mention that the ground on the input side needs to be isolated from the ground on the output side or this will occur, but it seems to me an a car all grounds end up tied together through the chassis and back to the battery; so how would you isolate them? Clearly there is something I'm missing here. Could someone 'splain it to me please?