The kernel does send out some messages during bootup, if you don't have car_serial_rate and notify, the player shouldn't be sending anything. However I don't think either of these would 'reprogram' a receiver.
The used protocols typically use non-ascii characters and checksums to avoid such things from happening. How is that receiver powered? I might be a power spike, 12V in a car isn't really regulated, so it can be anywhere between 9 and 16, (or even worse? maybe I should add a diode to protect my gps from negative voltages).