Ok I have finally started the project (alpine hifi kit). Today, installed the mid range and tweeters on both front doors. It was a bit scary pulling all those plastic clips, but they held on and so far no injury to the car. It allready sounds great with just the new midrange and tweeter added. The upper end soars, the midrange is clear and the bas is tight and louder. This is what the sound should have been like in the basic version! I used the instructions on this site to remove the door panels. It has good instructions with tips and pictures. It does not tell you how to remove the L or C shaped plastic trim that covers the upper edge of window and contains the tweeter (also called sail). It pulls out easily enough. Re attaching that L or c shaped trim was a pain in the ----. There is a tongue in that piece that is supposed to lie under the rubber seal and that is where it does not want to go! You will recognize it when you get there!
I guess one of the cheapest project would have been to just change the front door speakers with a non oem mid range and tweeter of good quality. The non oem tweeter can be easily attached to the sail on the L plastic trim. Drill a round hole in that thin plastic sail and push the tweeter in to wedge. The plastic trim even has a foam cushion behind it (in the E91).
Tomorrow, I am going to tackle the amplifier connections. My main worries are,
(1) How do I remove the back seat and back seat backrest?
(2) How do I remove the side cushion in the back seat to tunnel the wires to the front part of the car?
(3) How do I identify the wires to make proper connections? I just noticed that the Slovakians had misslabelled the wire going to the head unit of one side to bas speaker of that side!! The speakers and the harness is made in Slovakia and the amplifier is made in China.
Grateful for tips.