Maybe a broken wire, check the wires from control panel to heater valves, wiring diagrams are here
http://shark.armchair.mb.ca/~dave/BMW/e32/
also check on both sides of the IHKA control panel if the plugs are interted correctly, the control modul is farest behind the center console. Each side 2 plugs.
Read this, same issue
http://www.bimmerboard.com/forums/posts/690327