What's a good, cheap (really cheap) way to get decent sound into a
Nissan Pickup.

Back in the day, when I had more energy, I had a 1975 Datusn Pickup.
I had a pair of 6x9 speakers that barely fit behind the seat facing up.
I tore one side out of the boxes so they vented under the seat.

Had a homebrew amplifier with a single pole and two zeros bracketing the
cab resonant frequency.
Also had tailored negative output impedance in the region of interest.
It sounded pretty good.

In 1990, I upgraded to a 1990 Nissan Pickup.  They'd changed the cab
design slightly and my speakers didn't fit any more.  So, I lined the
back wall with carpet padding to damp the most  horrible cab resonance
and stuck a pair of Radio Shack Minimus speakers on the floor back there.
Sounds like crap, but I had 15 years less energy.

I'm upgrading the 1972 radio, so thought I'd make a winter project out
of improving the speaker setup.  I don't want to cut holes in the doors
or dash.  I'm thinking about making a wedge shaped thingie with the
left and right speakers firing out the top and the rest as a subwoofer
firing out the bottom under the seat.

I'm interested in pointers to articles, designs, experimental results
for things like:
Dealing with cab resonances
What can be done behind the seat of an import pickup
DSP techniques for active removal of acoustic imperfections in the
pickup cab and speakers.
I have a low tolerance for carpentry rework, but enjoy
tinkering with the electronic part.

Again, I'm not wanting to spend a bunch of money.
I'm not needing the ULTIMATE sound.
Mostly needing an interesting project.

Thanks, mike