Hey this thread may not have anything to do with your heater issue but check it out as it relates to doing as much diagnosis as you can before diving in to your dash! One thing for sure about these cars there is always something new and interesting to learn and this forum can be the difference between learning the hard way versus the easier way, Good luck!

http://www.peachparts.com/shopforum/showthread.php?threadid=33244

P. S. on my 126 on the passenger side towards the firewall is a plastic kick panel, behind the panel are a couple of electro boxes, there is one with red connector plugs that you can disconnect to safety the air bag before you play with it, you car may be similar.