Hey there Tif, i had exactly the same problem as you, and i bought a blaupunkt underseat sub, and to be honest it sounds great. Enough bass to rattle the windows if need be, but for normal day use it is perfect.
It cost £100, and then i paid £60 to get it professionally fitted, which in my opinion was a bargain as the guy made a perfect job of it.
This is the link...
http://www.bluespot.co.uk/stock/spkr0_2.aspThis is what it looks like:

It is under the passenger seat, some girls like it...

And then others just think it sounds great. But lovely sub overall, hope this helps.
Jonnn