Thanks, Martin. (As an aside, I removed the original door lock module since my solenoids are still working very strongly and I wanted to keep them that way. My plan was to install a lock module like Rob has done, with remote keyless entry but without openers.)
If I understand this correctly, both motor legs of the locks are normally open, and the switch connects alternately to both +12 and ground/earth when engaged? Is this why the solenoids have two leads instead of just one? So for example when I press the switch on "lock," it connects +12v to wire A and ground/earth to wire B, and when I press the switch on "unlock," it connects ground/earth to wire A and +12v to wire B?