Under seats?
Is someone smoking crack?
If its an off axis tweeter aim them right at each other from the sail panels. The A Pillars will work also, but it also raises the sound stage.
If they are on axis then aim them towards you.
Most tweeters will be off axis though.
Flush mount is lame. I took the stock tweeters out. Used a dremel tool with a sanding drum and fit my Rainbow surface mount cups. I ground away minimum material. I used some hot glue to get the desired position. Then a dab of CA glue. Then I pulled them off and souped them full of Epoxy. Then a light fillet of bondo. Minimum sanding- flat black paint.