A3 2011 Door speaker

My question is similar to den9112 below.

Bought an Audi A3 2011.  Drivers door speaker or tweeter not working at all.  Recently sold a van that I fitted a good sound system in so one speaker is just not going to cut it for me...need to sort this.

My questions are;

1. What is a good replacement speaker for the Audi A3 (2011) that will fit directly?
2. Are the tweeters easy to get to and replace
3. If I were to install a Concert head unit and add in rear door speakers, is the wiring already fitted or is this a much bigger job?

Thanks