1. Make sure you have the backup input enabled in the settings. I have an AVIC-X920BT, not AVH-P4200DVD, so I'm not sure how to enable it, but on my model, there is a setting to turn it on or off.
2. Make sure your connections are solid, even in the wiring harness. If your bypass is the same as the 920, make sure the wire you moved is inserted tightly into the harness.
3. What did you connect to the reverse light, the camera power lead, or the violet/white wire from the head unit? You need to connect the camera's power lead to a 12V ignition source, and the violet/white wire from the head unit should be connected to the reverse light circuit. I don't think the bypass is messing you up.
There is a note in the installation manual
, on pages 7 & 9:
Of the two lead wires connected to the back lamp, connect the one in which the voltage changes when the gear shift in in the REVERSE (R) position. This connection enables the unit to sense whether the car is moving forwards or backwards.
So you should make sure you're tapping into the correct wire of the backup light circuit. Put a voltmeter or continuity tester on it to verify.
Finally, I'm sure you already know this, but if you still have problems, post your question at avic411.com