I have the glove box out and the underdash panel off and can see all the vacuum motors for the doors, but I don't know where the heater valve is. I often have to slide the temp control back and forth several times to get it to change from cold to hot.

The temp valve is an electric motor, not vacuum controlled. It's basically triangular in shape, and is mounted on the passenger side of the console just behind the dash.

Here's a picture of what you're looking for from AC-Delco's website
http://webcaps.ecomm.gm.com/internet/ViewPartImage.jsp?acdelcoPartNumber=15-71846

Thanks!