I just checked it again. The computer boots (insanely fast, I love SSD) plugged into a PSU ( 20pin, with p4 connector attached) on the bench. However when attached to the car the light on the mobo comes on when I press the power button (PSU in dummy mode) and I hear the power "wine" but the fan does not start, nor does the boot sequence. Without the p4 connector on the board, the CPU fan spins up, then powers down and no boot.
I think the mobo I have is drawing too much power at start up, I guess I'm going to spend a little extra and get the dsatx