There is no real answer unless you know am SQ guy who knows your car inside and out. And even then he may or may not know the best position because people are different heights, and therefore is either sitting closer of farther away from the dash.
Best advise I can give is to sit in your car how you normally drive and roll the windows all the way up. Then using double sided tape or velcro, mount the tweeters anywhere manageable and take a listen. Even though you will have exposed wires for a little while, it will be well worth it in the end knowing the tweeters sound how you want.
A general rule is to mount the tweeters a couple inches higher than the highest point of the dash (if you're mounting them in the a pillars). Many people get good results firing the tweeters directly at each other. Others use deflection to their advantage but thats hit or miss in each car.