Chevrolet: 2003 Chevy Venture, chevy venture, cab air

Question
I read your response to the 2002 Venture no heat and it is similar to my issue, however, the cab air filter doesn't jive.  My 2003 Venture is fine and puts out heat while driving (in motion).  But when the vehicle is not moving, the air temperature immediately cools down.  It started a few months ago with the rear being slow to warm up and now it is this way with the front.  Do you still think it would be the Cab air filters?  The engine temperature gauge seems to act normal in terms of heating up and staying in control.

Answer
David;
HI  if you have heat blowing out  and the temp. gauge is working , if the front heat is good but the rear might be closed or check to see if any vents are shut off,etc.
I would check the heat line going to the back  and the coolent.I do not know what you mean by cab air filter??