You'll need to remove the drivers door card to be certain Ian but i'm fairly sure you'll find something like this in there :
https://www.ebay.co.uk/itm/CENTRAL-L...Y/230782233283
Chances are, it's worn out and the original locking mechanism won't work anymore because the solenoid has gone stiff. For the cost, you might just as well try a new one but also lube the latch on the door as well.
Once you've found the aftermarket actuator/solenoid (the drivers door never had an OE one on early 940s, only later models, likewise the passenger door didn't have the switch for the locking/unlocking), if you remove the two self tappers so the whole assembly is free to move up and down, you should be able to lock and unlock the drivers door with the key and also by pressing the lock button down and pulling the door release internal handle to unlock.
The Gemini alarm/immobiliser was an (Thatcham Approved Cat1 if memory serves) aftermarket system, usually fitted by autoelectricians and/or car security specialists. Was a good system at the time, not cheap either!
Had a similar problem with my 760 recently, a bit more serious for me as the previous owner lost the original key so the aftermarket solution was fitted with a secondhand ignition barrel, leaving all other locks inoperative with the key!
