[Shotimes] (OT) audio advice, please: adding a sub to 94 CV

[van Oss]

My oldest son is driving our 94 Crown Vic.  He's expressed interest in
adding more bass to the audio system.  That's what kids do these days.   : )
His 17th birthday is coming up, so I am exploring subs, poking around
FleaBay, etc.  Two things you should know:

-- My 92 SHO has factory JBL (two amps, one 6" sub) with 4 aftermarket
speakers.  The quality and volume are more than enough to satisfy me and my
son.  In fact I don't want him to have that much volume.  I'm looking for a
qualitative improvement, not quantitative.

-- The CV has a $100 aftermarket CD receiver which we think has RCA
connections for a sub.  The front-door speakers are factory but the rear
6x9s are 2-way aftermarkets.  The present system contains no amp.  He
listens mostly to pop and hip-hop.

As always, budget is limited.  If you had $100 to spend on this audio
system, what would you do?  Front speakers?  Main amp?  Powered mini-sub?

Also one specific area of question:  How do subs get sound into the cabin?
I mean, the JBL sub points the sound through an opening in the rear shelf.
The enclosures and tubes I'm looking at, it's not clear --- do they also
want a hole in the rear shelf?  Or does the sub just sit in the trunk,
sending rumbles through the rear seatback?

TIA for any advice.
VO