A while back I thought my entire HVAC system was Kaput. I could only get heat via defrost. IT had happened after a short stop when everything had been working fine. In my 1980 (I can't speak to the effectivity) If I blow one of two fuses that service the HVAC, the system kicks into a limp home mode of defrost. Not a bad design. You might check all of the fuses associated with your HVAC, including rotating the fuse physically in the holder. In my fuse box there is an additional fuse in a seperate holder (if memory serves).
And it is a B&*CH to diagnois the 107 or 116 without the book. At least for this shadetree mechanic.
S, J.R. Brown
1986 280GE LWB Anthracite
1980 450SEL Champange
1981 Volvo 240DL UPS Brown