Before you pull that busted blower motor, make sure road grime, rust, paint, or some other obstruction isn’t keeping it from getting the voltage it needs.
If the motor has a black ground wire, follow it from the motor to its terminus, looking for anything that might impede the connection. If the motor has no ground wire, then it uses the motor mount as its ground. Either way, if the mount’s ground path is somehow interrupted by rust, paint, dirt, etc., then the blower motor won’t be grounded, either.
Use a multimeter to make sure you’re getting full voltage to the motor. Once you know the voltage is adequate, then you can then look for worn brushes, dirty squirrel-cage fans, damaged fan blades, and other typical motor problems.
If you’re still seeing voltage-drop, make sure the ground wire is the right gauge. Our Red Dot Wiring Pocket Guide (RD-5-12560-0P) shows the maximum wire length before 0.5 voltage drop occurs, as well as the voltage drop through various connectors.