The mp3 mod means that u have to take your radio out and solder a connection onto it. It's very fiddly. But can be done in about an hour.
To get the radio out means removing most of the dash from the front. It's a pain if u have never done this before.
Go the tescos, buy a tape adapter and just leave it hanging out the front of the boss unit.
Where are u from? I could prob do the install, if ure local.