I'm just starting off building up a AT power supply replacement (Xbox) to go into a friends car. I'm familiar with h bridges, AVR programming and car electrics but no expert. I'm throwing this out there and hoping for some constructive criticism.
At the moment I'm thinking a linear regulator (7812 + pass transistor)for 12V (~3A) rail with fastPWM controlled switching into LC low pass networks, independent for the 5volt (~10A) and 3.3volt (~5A) rails. The PWM would be fuzzy controlled though a very heavily filtered/averaged ADC. The electronics would run off a 7805 with good heatsink and the entire enclosure would be shielded and well away from the engine bay. I have some mega8s lying around so I'll use one of them.
In general Xboxes are very tolerant of thier power supplies so there is little chance of catastrauphic failure.