I have a 2004 2500 diesel and I am having cab heat problems. I do have heat on all positions of the selector but it seems like the blower motor is not pushing out the volume of air it should be. Talking with a few people, they are telling me that there is a cab filter in behind the glove box. Is there any checks that I can do to see if the motor is bad or thermosister or something else. It's cold in Minnesota and need some heat.Thank you.

 

Sounds like your blend door has broken and laying on the intake for the fan. There are 2 ways to check this.1st is to remove glove box and look in back and see if you can see the blend door closed over were heater takes air for in the cab. 2nd is to take blower motor out and stick your hand up in there and feel around for the door. when mine broke I pulled the motor and was able to take the door out and reinstall motor.
There are lots of post on here about the blend door fixs.

 

Thanks, I will have to try that. I think it will be easier to take the fan motor out just because its easier to get at and working in a cold garage I might break some plastic on the glove box trying to get that out.

 

to look at the fresh air/recirc. doors, you have to remove the glove box or pull the blower. The stops on the top of the glove box door are the clips that hold it in. They shouldn't break. The fan motor is easily removed with 3 sheet metal screws. You should be able to look up or use a mirror to see the damper. There is no air filter.
If the damper is broken, you may want to 'jam' the damper, blocking the fresh air inlet, and, if you can get at it, disable that actuator that drives that damper. To make room to get at it, you will have to remove the passenger airbag
Repairing the damper requires pulling the HVAC box.

 

As said, common blend door issues. There are threads around with pics on how to resolve but Metaldad gives a good explaination

 

Just so we're straight, this is the recirculation, not the blend door. Blend door does just what it says, blends the air to give the desired heat or ac temp. The recirc door is either completely open letting in fresh air, or completely closed using only cab air(recirculating).
It is fairly easy to replace, and there are several write ups on the subject. Many have just glued it in place blocking the fresh air from freezing the passengers legs, but this results in reusing cab air, which in winter will make it very difficult to see out of your windows. Extreme humidity. I would suggest you replace it rather than glue it.
By the way, the recirc door can be replaced without pulling the dash apart. All access can be done through the opening of the fan motor, and removing the glove box. I have done it myself, following an awesome write up that another member posted.