The receivers in the doors and trunk are just that-receivers. There is a test in the diagnostic manual that tell you if they are working or not, but the "strength" is determined by the output of the key. The receiver has no "strength", it's thought of more as an antenna. These receivers can go bad though. If you are sure you have a good IR signal from the key, as determined by battery voltage in the key and/or testing the IR output with an IR tester card (available for example from Radio Shack for testing any other IR remote, usually advertised for testing TV remote controls), then you can thing in terms of replacing an IR receiver. In case of reduced range for the IR key (less than 10 feet or so) I would first suspect a weak battery in the key. Do you have another key to try? What is the problem with the remote locking system?
pa I think he was joking about the IR flashlight.