I added two 3 1/2" Kicker coaxial speakers in the dash location and I am going to add Kicker 5x7 coaxial speakers in the stock door locations to try and get a decent sounding, four channel separation sound as best I can with my fold-down rear seat car that I am not willing to alter the rear interior panels on at this point.
I am bevel mounting the 3 1/2' speakers with wood framing in the stock dash location and am going to cross phase them. I have done this on a couple Firebirds that I have had and most recently with my Camaro that all had center mounted dash speakers like our cars do. Basically the speaker mounted on the left is tilted slightly to the left but it is wired to the right channel and the right speaker is tilted slightly to the right and is wired to the left channel. The left speaker is tilted so the sound bounces off the glass like it is coming from the right and vice a versa. This helps the two speakers, that out of necessity are mounted right next to each other, gain some separation and that helps with imaging. Being aimed away from each other the sound can bounce off the glass separately and not just mush together. Although I know it would sound much better if it was possible for the speakers to be mounted outboard away from each other. Considering that is not an option to do that and keep a stock look, this is the best solution I could come up with.
It did help with my other cars when I have mounted the dash speakers this way. In the Firebird that I initially mounted both speakers heading straight up right next to each other, the sound seemed to blend together. When I remounted the same speakers in this way, I felt the separation was improved, so this is how I install the two dash speakers in the one center location since.