can anyone provide me the technical details of transmitters and reciver used in car central locking systems.
i just want to know on which frequcncies they operate and what kind of modulation techniques are used.
i lost one of two remote transmitter of my car. can i build a transmitter with the help of the one which i have?
can u provide me brief details how does central locking tx and rx works (block diagram) and what type of encryption is used for security.

Likely cheaper, and much, much easier to get a replacement from a dealership. There is a reason why your remote only unlocks your car in a packed parking lot, and not half a dozen similar ones. Don't think the details are easy to find, sort of defeat the security aspect, since you would be building sort of a 'universal-remote' for cars.