My 964 has two subs built onto the vertical face of the rear seats. To help you picture what I mean, if you were sitting in the back seat, one of the subs would be behind the calves of your legs.

The boxes are made from plywood, and covered with carpet to match the car.

.. Gregg

 

Iam 100% that a swd1600 fits under the seat of a 964, not completley.
But most ov it fits under.
I had mine under the seat with standard seats and with the PP,s.
Ive posted pics previously.
Not as good as a sub in a box, but makes a big difference.

 

Found old pic, i think i pulled sub out so you can c it better.
Defo it fit under at least 50+.%

 

I'll take a pic over the weekend of my sub in the front trunk. It does sound really good!

 

Go custom box / rear seat delete. Mine extends down into the footwell to increase volume. You won't regret the sound a good size good quality sub can give the system. I questioned whether a small sub would do the job, but I like my 10" JL.

 

I had a bazooka tube in the front trunk. It worked pretty good, a bigger one might have been better as I had the smallest powered one. I took it out as I wanted the luggage space, and tended to listen to the engine more than the radio.