The 2 12" L7, may work, but I had the same problem with sound with other set ups. But with the 15" L7 6 cuft ported w/ 1200 watts works. It flexes the roof on some songs, which is hard to do in that car. Make sure you face the box upward though, it is significantly louder with the sub facing up and down. That set up slams, louder than others in hatches or anything, you have to go overkill in the grand marquis, that way when u loose sound, it don't matter becuase you save your hearing anyway and its pleantly loud.
A comparision is my set up in blazer, blazer is slightly louder. Just build the box right, set settings correctly, get good amp and you'll be set. i can give u my box demintions if that will help?