'94 Mazda MX3 AC problem?

My boy has a MX3 and the AC recently quit working, specifically the compressor clutch won't engage. He brought it over and I made some checks. When the system is turned on, the compressor clutch does not get current. Using a jumper wire, I can energize the compressor clutch, it engages and the system functions, blowing cold air, meaning the compressor/clutch is fine. I bypassed the high pressure switch on the firewall by the sight glass but that also makes no change, meaning that's not the problem either. The electrical relay in the left rear corner of the engine compartment clicks when the system is turned on, but swapping relays also made no difference either. Since I used a known good relay (but of different outside physical configuration), I also assume that's not the problem.

When the system is turned on (but the compressor is failing to engage), checking the 4 wire electrical connector slightly forward of the sight glass, I get battery voltage on the white-brown (I think, can't remember all the wire colors), but zero voltage on the two wires that are the same color as those on the high pressure switch and zero on the remaining wire. This 4 wire harness is the one that goes through the fender skirt and into the cabin it appears. My boy called Auto Zone and checked on a low pressure switch, but they said none is listed.

Any ideas of what else to check? Is there a low pressure switch somewhere? I'm familiar with AC systems, although this is the first Mazda I've ever had my head under the hood of and the boy's Chilton shop manual is worthless as teats on a boar hog. All it says is to take it to a professional. Any help wouild be appreciated. Thanks!