Jim,
It's a 12 volt. I found where the resistor sits inside, and yeah, it's a fun one to pull out and put back in. It seems to be working fine, it gets nice and toasty down there, but it the heat just "hovers" inside the unit instead of warming up the cab. It sucks cause it's pretty chilly out this time of year out here in Chicago. I hear the fan going, but there just isn't much output as far as it is concerned. I wonder if the blower motor is not getting enough juice. I tested it when I bought it and it was getting 12volts. It was suggested from my neighbor that I may have put the fan wheel on backwards, but I can't see any other way to put it in there without issues. I put it back the same way I found it, the blower motor should be sitting "inside" the wheel correct?
