why would you want 6x9s under the back seat???
if you want a fuller sound get pro audio speakers to replace your current speakers..
as for under the 2nd row. i know plenty of ppl with 2 12s under the back seat so you can get 6x9s to fit. i just installed 2 jl 13s (shallow mount) under the seat in a friends yukon. tight fit but it fit
lol me and you are gonna box Drop.. lol..
Although i agree with prefab boxes out there for 2 8s for a dime a dozen.. building a box for a pair of 6x9s under the seat sounds a little.. unnecessary.
But on the same note.. i know for a fact that 4 BAD ASS 6x9s WILL fit under the second row.. and be a full excursion speaker and will ROCK 2 shallow mount 12s out of their socks.
But then again im NOT talking about a coax or 3 way 6x9 im refering to an actual Sub.
I personally wouldnt waste the money on jl shallow 12s (in this case) since full excursion 10s will sound just as good for 1/6 the price.. or even running JL 8w3s would be a good option.