I just got my restoration project back from the upholsterer last week who made kick panel boxes for 6" speakers + tweeters in the front and I had two 10" subs installed in the parcel shelf. Personally, I would not recommend installing them in the door panels; I have that in a 250sl with that and they really look bad. Also, it's space limiting. IMO, the more hidden the better. The previous owner had two speakers installed in the real wall above the parcel shelf where the soft top goes. I've never heard them installed there, but I don't think the sound would be very good, they would be very visible, and it would be space constrictive for the top...lower down on the "wall" would likely work though. The car is at the shop, but I can post some pictures tomorrow. Original becker radio with amp under the seat. The sound it pretty darn good!
Pic attached.
Tom