Okay, first things first. Get the OBJ file re-downloaded with a proper download manager to make sure you're not blowing damaged code into the chip.
Then... find out what serial port speed this custom interface is supposed to be working at. If it's running at 4800bps, that might be your problem. When the player isn't docked, the speed is 115200bps not 4800.
Also, the wiring of the serial port itself is different between the home home connector and the car docking connector. On the home connector, it's "straight" so you need a crossover cable, on the car docking connector, it's already "crossed over" with TX/RX swapped, so you need a straight cable.
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