I have the same problem on my early 01 ram. Its out of warranty (43k miles), and when i was at the dealer getting the power steering pump replaced i asked them to quickly diagnose my problem. The seats just dont heat up, the power seats work just fine though. Its kind of wierd. On *some* days it will work, but on most days it will not work.
I found out that sometime in 2001 they changed the heated seats style. I guess in early 2001 the heated seat part was SEPARATE from the seat itself. In late 2001 they integrated them together. I was told from the dealer that it would cost approx. $200 for the heated seat part plus about 2 hours of labor (have to completely remove the seat to get at the part & replace it.
If it were to be the late 2001 part it would be approx. $500-$600 plus about 2 hours labor.
So needless to say, i wont have heated seats this winter! Maybe next year. That, or i'll get an espar heater for a few hundred $$ more!
Tim