S E A R C H H E R E
I don't think any head-unit would have an IDE or SCSI controller where you could plug in some harddisk in.
Then you need the ability to access the disk.
and then you need hardware in the headunit which is able to decode the stream of mp3-data into audio..
Search around this board.
I would also recommend to take a look at the examples pages many of us have made. You can find them HERE.
Just look at those pages and give yourself the time to develop the idea.
If you encounter problems then we are more than happy to help you, but one just has to get some 'basic' info about in-car-computing before one can begin with this project.