2004 Kenworth W900 L blower motor broken/issues

Okay so here's the issue. A few days ago my blower motor quit working on any setting (including high). Tore the box apart and found I had 12 volts at the connector (actually had 12 volts on all speed settings). Got a new motor, hooked it up and all was well. Worked fine on low medium and high. About 5 hours worth of running later I noticed the motor getting weaker. Turned it to high and could feel a noticable drop in speed. A few minutes later, nothing, dead. Now as I am writing this it started working again but weak on each respective setting. Any thoughts? Sure could use a clue. Thanks in advance.

 

Forgot to mention, hooked old motor direct to batteries. Nothing, dead as a door nail. Again, thanks for any help.

 

Big thanks to everyone who responded. I took it all back apart and cleaned the ND dust out of everything and reassembled. Seems to be a bit better. My only follow-up question, should the resistor read 12v on all settings when the motor is disconnected? When the motor is hooked up low, medium, and high all work fine but when I disconnected it and tested it always says 12v. Doesn't seem right. Thanks again for all the posts, very helpful.

 

Thanks Gear head, makes sense now.