I wrestled with this for a while. I didn't want to lose the standard looks.
Jay's solution, though not the simplest was well carried out.
http://www.omegaowners.com/forum/YaBB.pl?num=1153396039/0You'll be able to fit speakers with mounting depth up to 63mm if mem. serves. That's most good quality ones such as these.
http://www.caraudiodirect.co.uk/product_info.php?products_id=3020&cadid=136305922e8ae11c460dd1dc6bb8099bBe careful of magnet diameter though or you'll be attacking the aperture in the door skin with a hammer!
The other way is to use the speaker wires to drive high level inputs to an amp. (Most amps have them).
Option 1- Small amp and sub or active sub. Original speakers high passed, running off headunit. Very effective upgrade.
Option 2- Four channel amp. Two channels running fronts (components or standards) and two channels bridged to run 10" sub. 'Normal' setup.
Option 3- Two amps. (Like Jay). One running components. One running bigger subs.
A lot depends on preference, pain tolerance (It's awkward and fiddly fitting this stuff) and budget.
How much is Mrs Markiec prepared to spend?
I can recommend option 1. My freind has a 100Wrms amp and 10" sub in his Passat estate and it sounds very good. A good compromise of cost, effectiveness and ease of fit.
Bear in mind that using high level speaker inputs to the amp, you're going to lose some quality, but I'd be surprised if you could tell under normal conditions.