Although it has nothing to do with this thread, there are GSM services in the US from Powertel, PacTel, Nextel, and others. The fundementals should be the same regardless of technology, you need to connect to an aux speaker and mic, and finally to an antennae. Today's mobile handsets are great, but I think each one may require a different approach to installation in that they do not share a common standard for speaker output impedence or microphone input level. My thought would be to buy a car kit for the phone of interest, determine the wiring connections, and "splice" it into the existing wiring. Not trivial, but I do not think impossible.
94 E500 (Silver/Black)