Weird Heater Motor Problem

I need help!
'94 cherokee, 4.0 HO. just bought a few months ago. truck needs work.
OK so here is the history. i read thru many pages of heater info.
I had a heater motor that would only run on high speed. it squealed badly.
pulled the resistor. it was rusted.plug was OK. replaced resistor. motor ran on all speeds after resistor replacement,but still squealed, & resistor started to smoke. shut down quickly.
pulled heater motor. it would barely spin by hand. replaced. tested motor & resistor prior to full install,motor spins great, runs on all speeds, but after a minute on low fan speed resistor heats up & starts to smoke.
So my question is - after replacing bad resistor & motor(not installed, just plugged in), what would cause the resistance to heat up & burn out resistor. again resistor plug looks great. I may be having a ground issue, new wires from Jon (5-90) are on order. thanks for your help.

High amperage. You cannot destroy voltage in the system, only use it in a different way. Your resistor turns electrical energy into heat energy so it is going to heat up especially on low because it is absorbing a lot of voltage. Smoking however is not normal. Check the amperage draw of motor and resistance of the circuit because if the motor is hard to turn mechanically it draws more amperage which causes more heat inside the resistor.

would i have to worry about amp draw with a new blower motor?

A quick note;

The resistor is air cooled and relies on a positive airflow to keep it cool enough not to burn out, that's why they are mounted inside the duct. Many of the newer ones are nothing more than carbon paint on a ceramic base. I would not run the components long outside the housing.

X2. Resister needs to have good airflow over it to survive.

Just because it is new, doesn't mean it is good. HOPEFULLY it is good.

Feel the other components like the switch and plugs for heat buildup which would show excessive resistance.

Good point about the resistor needing airflow.
My plan is to get a new resistor, & pull the dash to inspect the speed switch & plug,& try again. I think i may be able to get gy switch & resistor to try.