I think the problem is that the OEM switches go through the smart junction block so they aren't "real" switches like the rocker types that come with the heated seats.
if you're PCM isn't programmed for heated seats, you may have a problem trying to get them to work off OEM switch.
did you check here: http://iihs.net/fsm/?dir=40
I recommend the Dormer heated seat inserts. you can get then from advance auto and use an online coupon code to make them much cheaper.