is just a standard female-female 9 pin (not the one that came with the riocar),

And there's your problem. You need the one that came with the rio car. If you don't have it any more, then you need to go to Rat Shack and buy one. It must be a NULL MODEM cable.

If you want to know how to tell a null modem cable, the pinout specs are:

Pin 5 to Pin 5.
Pin 2 to pin 3.
Pin 3 to pin 2.

Those are the only three wires used by the car player. Note the first pin is the same, the last two are "crossed".


ALTERNATIVELY...

If you happen to have the docking sled, your straight-through cable might work with the sled. Its serial port is "crossed" already.