My Renault Fuego was the world's first production car to have central locking, but as an Infra-Red based system at 25 years of age, I knew of only one that still worked, and that was achieved by standing next to the door and aiming carefully through the window at the receiver in the rear view mirror, hardly the ideal remote.
I designed a UHF radio based system with a 300m range that I went on to sell about 20 units to members of AussieFrogs and even added it to other makes such as Citroens. In those cases, I wired a receiver relay into the internal central locking switch.
Let me know if you're interested and I can talk you through the idea.