Well it's quite simple. You just have to poly crunch it from 500k+ to around 70k-110k. then you have to texture it, fit out a decent interior, fix up the gaps between the body and the interior, model an engine, boot and do general fixes to it. Sometimes you have to make a wheel too which is the worst thing in the world. Then once thats done you have to sort all the parts into the correct objects, create collision files for everything, then you have to compound all the objects into a game readable format. Once all that's done it's a matter of repeatedly going in game and ironing out any bugs, and changing anything you need.
Onto the emergency side of things, you have to model and convert the lights for it, work them out to work for the default gta lighting system (yuck), make all bits of gear you need, make liveries, bug test, and boom, got yourself a model.
For me that whole process is about 15 hours on an average model, although I rarely get time to work on a model for more than a few hours at a time.
confused you yet? :P