I've been reading and bookmarking quite a few links/sites for my upcoming install (picking up my MCS either today or tomorrow), but I have not been able to find an answer for how to best run aftermarket wiring from the front door speaker locations back into the cabin. I strongly prefer to avoid using the stock wiring, so that a future uninstall will be painless, but in all my reading I haven't seen much mention of installing wiring. From what I can see, there isn't much of an avenue to follow to run wires from the door through the A pillar. Anyone out there have some experience and guidance they can share?

 

Using the stock wiring is the best option. You can buy an adaptor that plugs into the connector that originally connects to the stock HU, and use that to connect to the stock wiring - then you can simply remove it when you need to revert to factory.

There is a plug/socket assembly between the door & car in the hinge area. You have to drill through this to run wiring - very painful :(
MikeyTheMini has a DIY for the folding mirrors, which describes the same sequence of getting new wires into the door (step 12 onward). His company also sells the adaptors I mentioned above.