As topic says, is this plug and play? E.g. would I be able to throw a mp3 player, dvd player at my MFD2? And where would one mount the bugger?

It's not plug and play because there are +ve and -ve power supplies to connect.

For mounting you are restircted by the length of the cable so it would need to be behind the head unit.

Is there any guide for it? I guess I could hook it up with some cables and have them sitting in the glove box.

Yes that's quite common. The carrier in the boot for the CD changer is a 3 x Single DIN unit so you can run cables to there and mount a DVD player or something there.