Heater not heating on 1990 GXE

[Cal Learner]

The good wife's GXE has been owned since new and given regular care and maintenance. Looks good, runs good, drives nice, for the most part. 150K on the clock. But a recent move from warm climate to cold climate reveals a problem, or maybe two related problems: (1) heater produces nearly no heat, and (2) temp gauge on instrument cluster says the engine isn't heating up to normal operating temp. Or rather, it may eventually warm up to normal temp, and then suddenly it's back down too low.

I'm guessing the second problem is related to a failing thermostat, and that's easy to correct. Do you agree?

But I don't really know how the heating system on these cars works. On my Mercedes, I'd suspect an inop monovalve not opening to allow engine coolant into the heater core. But I don't know what the equivalent is in the 3rd gen Max. Help me out please. All suggestions gladly accepted. TIA

[the converted]

Sounds like both of the problems are related to the thermostat. I'd just swap that out and then see where you are.

[Cal Learner]

Will do. That's a weekend project, if I don't get rained out. Thanks.