I have retrofitted both the MDI and the RCD310 into my Polo.
The RCD310 was easy. All I had to do was remove the surround unscrew the screws around the side of the radio and it popped out once you unplug everything.
The MDI was a bit more difficult. I bought one that fitted in the glove box. I took off the glove box door and the whole frame of the glove box. Made a harness for the MDI to sit in to the frame of the glove box. There is a blank in that frame where the MDI port would show through. I just popped that off and then lined up the port to the hole and then screwed the mdi and harness to the frame of the glovebox. I then sent all the wiring round the side of the glove box in behind the radio and connected all the wires together. Put the Radio back in and the screw all the glovebox back together and it's sorted
Fitting the MDI does take a while but it is worth it
Sorry if my instructions are vague or unclear!