To answer your question about undercover speakers, I assume you mean speakers under the package tray.
When I upgraded to a new system, I wanted more separation of sound between front and back than you get from the door speakers when they are mounted in the stock position.
It involved removing the air vents for the flow through system, but I never used that any way.
The sound is incredible with the position of the rear speakers.
Each is mounted under the package tray, one on driver's side, and one on passenger side.
Bob.