Where is the central locking module located in a 1999 9.3 convertible

Most likely under the right/passenger dash. Depends on the model.

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Your central locking is controled by the TWICE computor. It is located under the drivers seat. Also it is not user serviceable. If your problem is lack of response of the vehicle to your remotes it may just be a programing issue. If the failure is lack of central control when you use your key it is most likely in the door switch. If you have any further questions drop me a line and l will see what l can do to help. Good Luck