There are many many many posts like this.
first thing you will need is a carnetix 1900 which is what you will need to power your mini while it is in your car.
Second you will need a screen. You can use the xenark or the liliput since these are the two most talked about models on this forum. There are others that may work, but just check to make sure that they have drivers for osx.
third. you will need to determine what you want your carputer to do? Are you going to run it as a windows machine or an OSX machine?
Let me know if you need any other help.