I have a 1993 Plymouth Grand Voyager SE

I have a 1993 Plymouth Grand Voyager SE. I'm having trouble with the heat. I drive approx 30 minutes to work every morning and I don't get any heat. I've watched the temp gauge and it barely moves during the entire trip, staying pretty much all the way left of the gauge. I've noticed if I sit in a parking lot with the engine running for 15-20 minutes, the guage starts to move and I can get heat. But if I start driving, after approx 5 min. The guage starts to drop back to the "cold" side and the heat goes away. I've never had a problem with the van overheating, and I checked the antifreeze level and it's full. I have VERY limited automobile knowledge, but would appreciate any help I could get.