Hi,
Sounds to me that the cars 12v system is positive ground,
and the door-locks want the newer negative ground.
The suggestion to reverse the supply to the door-lock
system is unlikely to work, because the return wire on
almost every system (except some military vehicles) is
the bodywork of the vehicle, in fact a lot of vehicle
parts are made so that their construction is such that
only one wire feeds them, and the fixing becomes the
return wire. To make insulated fixings and provide a
separate return feed would be possible maybe.
I would consider reversing the vehicles polarity, unless
there is a good reason not to do so.
John