you can make a simple circuit from 3 MAX1771 from maxim. it you put them in paralell with a common input from the battery, common output to the computer power supply, and a common ground, you will be able to supply 12v at up to 6A... if you want more design margin... just add another circuit in paralell and you will have 8A.
if you look at the data sheet you will see sample circuits with parts listed... use the bootstrapped mode as it will give you higher output current (2A per chip) and will require higher input current, but with a car electrical system that wont be a problem.
thats about as simple as i can see it... i couldnt find anything at 12v out with higher than 2A output (running on ~12V in)
if im wrong... someone please correct me!!!! im far from an expert on electronics design...