I'm not going to add much to the conversation that hasn't already been addressed, really.
For a performance PC, liquid cooling is great stuff. It's a must-have if you're going to do serious overclocking, and adds a cool aesthetic to a "show-off" PC.
However I don't think a vehicle is the right environment for a watercooled PC.
The cooling benefits won't be all that great, since you're only going to cool to ambient temperatures.
If your PC is in the trunk, the ambient temperature on a warm summer day is going to be too high for liquid cooling to be effective. That said, air cooling under those circumstances won't be much better.
It doesn't matter whether you're pushing warm liquid across the CPU or blowing hot air across the CPU, the net result is the same: You're not getting the sucker cool.
The main reason I would recommend air cooling over liquid cooling in a vehicle install is vibration. You don't want a hose from your water cooling rig coming loose and spewing coolant on your electronics, especially while driving. Vibration is what can and will work the tubing loose from the fittings.
That said, the trend now is low-power CPU setups (like Intel's Atom) that don't generate much heat, many of which are passively cooled. This hardware will fare much better in a trunk install than a "standard desktop" counterpart. It's no guarantee that it won't have problems on the hottest of days, though.
doncarbone, I have yet to see any info on what PC hardware you are looking at. I searched your threads and came up with naught.
The hardware you choose will really determine the cooling needs.
That said, your diagram shows a pretty inefficient airflow setup. You have the fans pushing/pulling air around a 180 degree bend. Pulling the air through that isn't much of a problem, but the fan pushing the air (the 120mm on the right) will simply blow the air at the side (the inner fender wall?) and the air will stop and disperse a bit. Think of aiming a water hose at a wall, and you get the idea.
Of course there is! Links, if you're interested:(is there water cooling for the drives?)