I've got 1 10" under the dash, at the drivers feet. One 10 is not enough, I am looking to upgrade. I am not sure if 2 10s would fit? Or if I'll have to go with 1 12. But as far as location, under the dash is the only play to put a sub.
I've got 1-15" solobaric (square) kicker driver under the dash of my DLX. It does the job rather nicely. Although, if you listen loud enough you can make the guages bounce. I'm sure that's really good for the electronics.
I had a single 10 but then I got JL Audio 12. Also under the drivers dash in my 96 DLX. It hits hard enough for me but I dont have golden ears anyway. Evan is right it vibrates the dash and gauges pretty hard if I push it. Cant find anywhere else to put one though.