Ive seen this product mentioned before in these forums, but not for this reason. Its the "X2VGA 2" (http://www.x2vga.com/) box. It is a VGA pass-through and composite to VGA converter. So far nothing special, but when reading the manual (http://www.x2vga.com/manual/x2vga2_manual_eng_100.pdf) it might actually be able to passthrough VGA whilst off, and autoswitch to Composite 1 when turned on.
Their site says they are bringing out a new version at the moment, which means they are actively developing it. Theres a possibilty that with enough interest/demand, they could make a custom version for MP3car members which is exactly what we'd need - no mods neccessary.
Have a read of the manual and let me know your thoughts, or if anyone already has one of these, have your tried it as an auto switch device?
My only concern is that the power button needs to be pressed (for 2 seconds) even with 12VDC present for the thing to turn on.