I have a 2002 Dodge Ram 1500. I have no heat. I have replaced the thermostat. When my truck is warm I can turn on the heat and it will blow hot air for a few seconds and then begin to blow cool air again.

sounds like a blend door stuck shut under the dash. check fuses first then connection to door.

make sure the coolant level is up.
with a hot engin check the heater hoses, they both should be hot and the same temp.
if not the heater core is restricted and should be flushed.
if both cold, check for flow, disconnect one hose and start engin for a second, should have slow full flow of coolant, if not pos. bad water pump.

These trucks have been known to have blend door problems. sounds like yours opens but closes after a short time. may be the plastic blend door itself. there is an aftermarket metal replacement door available.

HeaterTreater is the world's leader in aftermarket blend door products and there is a stainless steel replacement interposer available for the 1500. Additional diagnostic information and pictures are available on the HeaterTreater Ram 1500 listing or the web site at www.heatertreater.net

Also check out this forum:
http://forum.doityourself.com/archive/index.php/t-367749.html

if it turns out to be a heater housing problem and you need any info on it let me know, i'm a master tech at a chrysler dealer and can send you any thing you'll need.
mws919