Sorry for not responding before now - did get your mail, but I've been seriously busy.

A few things to try:-

First off, trying wiring the sub 180 degrees out of phase with the rest of the system, ie. reverse it's polarity.
If that doesn't work, trying firing the sub towards the back of the boot (assuming it's not already). This is a good idea in general anyway, as you increase the path length to the listener, resulting in lower wave propogation. f that doesn't work, try positioning the sub so it's about 30-60cm from, and firing directly at the corner of the boot compartment.

If none of the above has any major effect, then you're going to have to somehow increase the airflow between the boot and the passenger compartment. If there's a 'ski hatch' option on your model, but not one fitted, then quite often there'll be a metal panel that you can remove in the centre of the back seat (behind the seat squab) without effecting structural strength. Failing that, about the only other option is to somehow cut ports through the rear shelf, then cover them with acoustically transparent cloth so they're not visible.

With the latter physical alterations to the vehicle, there is a slight danger you'll increase the amount of road noise entering the passenger compartment, but it shouldn't be too significant, and could probably overcome by a layer of Dynamat or similar applied to the boot floor.

Dominic

Cambridge Car Audio
http://www.cambscaraudio.co.uk