I have a Dodge Ram 3500 with a heater blower motor working intermittently

I have a Dodge Ram 3500, 5.9L engine, 4×4, manuel transmission, with a heater blower motor working intermittently.  It has now quit working.  I have changed the heater blower motor and the heater blower motor resistor.  It still does not work.

Is this a common problem as I have 2 friends with the same problem? 

Please advise!