1992 g30 heater motor problem

My heater motor is not working. I did the following. Checked for power at the motor and got 12.8 volts. So I figured the motor was bad took the motor out and decided to just check it. I hooked it up to my battery charger and it worked. I reinstalled it, no good again checked voltage and had 12.8 volts. Any ideas would be appreciated.
John

Is that voltage you measured WITH the blower motor plugged in? or with it unplugged?- makes a big difference- you may have bad contactsin relay that can show voltage, but not pull the amps needed to run a blower motor- if you measure o volts with the blower plugged in, then you have your answer.