Thanks everyone for all your replies. But maybe I need to rephrase my question. I fully understand that the things I'm trying to do are no replacement for a real sub in the trunk.

But this is a leased company car, so there's a limited amount of tinkering I'm allowed to do.
And I will immediately lose the car if I ever lose my job.
And I also quite often need the all trunk space available.

I already have the Canton speakers in the front door which is a vast improvement (an they install very nicely too).

So, given that subs are out and the only option is to replace factory speakers, what would you put in the rear doors:
1) 6.5" woofers
2) 6.5" full range speakers

A 6.5" speaker is not a subwoofer, no matter what it says on the box or in the advertising.

Well, I never called them subwoofers. I've called them woofers and kick bass speakers.

So yeah, I agree, a sub in the door, one would prolly not sound very good, due to all of the vibration that could occur, and two, they aren't designed for it.

Vibration is definitely one off the things I'm afraid of. But these kick bass speakers are actually designed to put in a door or a doorboard. But they should really be assigned the 80 to 200 Hz frequency range, according to an article in Autohifi magazine (6/2000).

I won't be able to do that, my amp can do low pass and high pass, no bandpass. Passing 0 to 200 Hz would be my only option.

Pim