I want my TARDIS to look like a 57 Chevy, and you'd climb in through the trunk.
1. You could climb into another dimension by climbing in the trunk or folding the seat down. (Now that's what I call trunk space!)
2. You could drive it places, or over gangs of angry primitives who would have carried your old phone booth up to the mouth of a volcano already.
3. With a TARDIS heart as your power source, you'd never have to fuel up.
4. If you're about to regenerate, it's not a problem. You don't have to trudge up to a castle to get healed. Just drive there.
5. In any year after 1957, nobody would blink at you tooling down the street in your TARDIS. In fact, you'd get lots of jealous stares, and you could probably even sneak past the radar during the 1940's or any era after Henry Ford becomes successful. Before that you'd have to call it a carriage, and possibly do an A-Team style chop job on it to blend in.