Vacuum central locking in your car is enabled by an electric pump. With a remote system that features extended lock/unlock signal pulse and information in the archives on how to wire a couple of relays, you can keep the existing lock system for a lot less than ~$200 for solenoid lock motors and ~5 hours to install them.

Crutchfield has or used to have relays prewired for this function.

Sixto
95 S420
87 300SDL