An '89? I was trying to replace the bulb in my HC panel and couldn't get
the damn thing out of the dash, because of the cables!

Unfortunately, the manual I have does not go into any detail as to how the
valve works at all! And, it's dark and raining right now, so I can't look.
I believe the valve is at the back of the engine, and not mounted to the
firewall as usual...

I don't know much about your '89 626 specifically, but most newer cars
(including my '90 Miata) do not have heater control valves. Perhaps it
was a cost savings ploy or a means of increasing overall reliability,
but they have now gone the way of the dodo. What happens is that coolant
flows continuously in the heater core and the proportion of heat that
actually gets into the cabin is controlled by opening and closing flaps.

Perhaps someone else can confirm my suspicion about your 626.

Ken