If you have the remote CD changer, it will only work with an Alpine unit of the same vintage, such as the CR-1. Some of us have installed Becker radios out of Boxsters, the CR-220 and CDR-220, have Auxillary inputs, the first Boxster radios the CR-210 and CDR-210 do not. The factory wiring harness is set up for Becker plugs, the CR-1 had an adapter harness out the back to connect to the Becker plugs. The existing stock harness plugs directly into the Becker radios. For over radios you will probably have to rig an adapter harness, no problem if you like wiring.
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