What i've found is if your watching a dvd, without the engine running, as soon as you crank the engine, the power drops and the dvd resets anyway. Maybe thats just my PS2 and the way it's wired in.

So if you were thinking this sleep timer would save you the hassle of putting the DVD back to where it left off then it may not help.

Other than that, this should do you perfectly - push the button before you turn the ignition off, then it will supply power for anything up to 15 minutes - Only costs £8 too so worth a try

Universal Timer Kit : Timer Kits : Maplin