300SD Heat Question

[NYDEISEL]

The car is a 1979 300SD (116). My blower motor is having the brush issue. I have to hit the area under the glove box to get the thing running. I have read many post here on how the brushes can be changed with parts from Ace Hardware. My first question is how easy of a job is this? My second question is with regards to tha actual temperature. When the heat first comes on it is real hot. Then after a while it gets luke warm. If I put the defrost on it gets real hot again and so on and so on. I know about the 123's and the mono valve but this is a 116 and I don't know what to do. Does this have something to do with the vacuum system? I ask because my door locks only pop up after I start the car to build up pressure and the doors have to be locked manually when I park the car because the engine is off. Anyone have any ideas??? I can't take the *****ing from the wife anymore about how her toes are cold. I love this car and will never get rid of it!!! Thanks all...