lol @ the "crappy v4 mobo" comment.
The V4 is much better than the V9. The only thing the V9 can do the v4 doesn't is have a built in IR sensor capable of power on/off, reset and eject the tray. (V9/V10 suffers of chronic cost cuts on plastics material and motherboard design. That's why I am saying the V4 is better than the V9)
Anyway, since you're hooking it to 12v, you can simply hook the PS2 straight to the car battery with no fear of having problems. You only need to consider filtering the supply with inductors so the PS2 doesn't send any noises through the power rail to your car sound system.
No need to spend a penny with DC - DC converters since the PS2 "phat" is already 12v. But a word of warning for others who might read this:
Really early PS2s (old japanese units) and all units of the SLIM range do have 8.5v power supplies.
DO NOT over voltage them as it can be harmful to the DVD drive circuitry.
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)