You need to pull the rear seat.  You remove the subs which are not in
enclosures.  You then use the enclosures for the 97-00 Jags, the HK premium
system, to mount a set of subs and mids and or tweeters.  These enclosures
are seen on eBay from time to time.  Jag sells them but they are expensive.
The fastening points are still built into your car.  Then you have some
choices about how to power them.  Most people pick up the outputs from the
amp by tapping into the connector under the armrest.  Fitting mids into the
enclosures appears easy enough but requires some hole cutting.  Jag used 2.5
inch mids, a very rare size.  You can always get them from Jag.  I'll
probably go that route myself.

I'm in the process of doing this on my Jag.  I want to replace the subs with
something better, and add some mid-range speakers.  Just waiting for a set
of enclosures at a decent price.  The problem is finding a set of 6 inch
subs. Really the only ones that you can get are JL Audio 6W0-4.  Even though
they are discontinued you can still get them new from dealers and JL will
accept special orders for them.

I suppose you could replace the subs with coax speakers but you'd have to
alter the wiring.  I am not at all sure how a coax would sound being fed by
the sub output from the amp.

Paul