[Nick850GLT]

I had a similar problem with my 850 Estate, the tailgate lock often failed to work with the doors. Sometimes it could be operated with the tailgate key - but sometimes that seemed too resistant to turn, as if something was jamming it. However, I could hear the tailgate lock servo whenever I operated the doors - it just didn't actually lock/unlock - from which I guessed I did not have a problem with wiring at the hinge.

To cut a long story short, the problem was a 'lack of power' from the small motor in the lock servo. The first photo shows the whole lock with the servo opened (a screwdriver and Stanley knife job!).
The next 2 photos show the servo fully open, and the failure with the opening incomplete. The motor has insufficient power to disengage the 'traveller' on the screw from the white operating lever (there are 2 spring-loaded detents).
The final 2 photos show the equivalent 'locked' positions, again showing how the servo fails to complete the operation and disengage the traveller. This failure to disengage the 'traveller' is the cause of the key 'jamming', as you simply can't turn it hard enough too disengage the white operating 'rod' from the 'traveller'.

At first sight, a new motor is required. However, it transpired that the problem was oxidation on the push-in electrical contacts on the wires where they connect into the motor. Simply cleaning them, and giving the motor commutator a dose of switch-cleaner, was enough to restore the motor to 'full power'. Now everything works again; long may it last!
By the way, I fixed the servo motor back using 2 M4 55mm long Pozi machine screws - and simply held the lid on by securing it with washers/nuts.

I spent sometime trying to track-down the motor. As far as I can ascertain it is a variant of the Mabuchi FC-280. However, so far I can't find the variant with the correct spindle - most of them come out the reverse end of the motor! The FC-280PC-SC is available with a 6mm long shaft, requiring an adaptor to be made. In the USA the FC-280-18180 is available with a knurled 9.5mm shaft (the correct length), but I can't source it in the UK.

I'm simply hoping at the moment that I won't actually ever need a new motor.
Volvo do still sell the complete lock assembly, part 9203375, at a mere £133.80p - but my repair cost practically nothing!

I hope this helps some others with this tailgate locking problem.