Since CAN busses need termination, you could try putting a 60 ohm resistor across CAN_H and CAN_L. The bus will indicate an error with no resistor.
If your CD changer needs to communicate with the head unit, you'll probably need them connected together. (It still may not power-up without being connected.) You'll probably want them connected to reverse-engineer the protocol anyway.
What CAN interface are you using for this?