My scanner is temporarily out of order and I have been using my radio technicians one recently. On Monday I can send the drawing but I have just reviewed the diagram (the good one is not in the climate control book). It is in the ETM. Totally unique wiring diagrams in 78-79. They are huge 19"x15", which makes them hard to scan. They will be the same as an 80 model.
From the diagram I can tell that you must have either a bad ign switch as Larry points out, a bad fuse five or connections, or a bad compressor vacuum switch. These are the only components before the compressor on/off switch. All of this is before the ETR switch or the compressor low pressure switch in order.
There are two major vac switches one knocks out blower, the "blower master vac switch" has a green vac line going to it. The other is the compressor vac switch and it has a yellow vac line going to it. They are both behind the glove box. The defrost by passes both vac switches. They can be by-passed electrically for testing but at least the blower switch shouldn't be left by-passed as it will make the system run on low blow even when the system is switched off.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician