Check if there's an aftermarket module to convert them to Sony stalk style interface, if so then just plug the output of that unit to the tuner module.

If not you'll either
* have to build a circuit to do some translation, probably to serial (and have it pipe the commands into the serial connector), normally a PIC one chip computer. This would also work if you don't have a tuner module.
* if the buttons are plain swithes and you can get to them, hook them up to a resistor grid and connect that to the tuner module (emulating a Sony stalk controls values) Requires a tuner module.
* something like the above but sending the values to an IR-transmitter that send remote control commands. Doesn't require a tuner, and keeps the serial port free for other things.

Probably not an exhaustive answer...

/Michael