As Hugo wrote, tuner connects to second 'internal' TTL-level serial port. However, you can't simply plug Sony stalk into 'regular' serial port: it is just a bunch of resistors; a simple ADC in tuner module reads the resistance and sends measured value down the serial port in the format documented on riocar.org. In principle, you can cobble together a little interface with a PIC, UART etc and do that yourself (somebody already did that in pre-tuner days). However, Pioneer or Kenwood IR steering wheel remotes are really simpler solution.
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Dragi "Bonzi" Raos
Q#5196
MkII #080000376, 18GB green
MkIIa #040103247, 60GB blue