Thank you for your reply, Pete. I will check the fuse this evening. My wife reported this problem to me at 9:30 last night, so I was not about to start a 'search & destroy' mission in the dark. If it turns out to be more than this, could you let me know where I will find the heater control module?
Last night I discovered that it was a blown fuse. Replaced it and the seats are warming again! Thank you for your help on this. I will gladly rate your service now that the problem is solved.