I just went into my local Halfords and saw one of these JBL units it seemed small enough to fit behind the seat (haven't checked the measurements yet) and because its already got the amp built in it makes it a bit easier for mounting. I think it was only £99.99. Its 100w RMS not sure if it was the Basspro or BassproII.
Heres the link to the JBL site for these. http://www.jbl.com/car/products/category.aspx?cat=sub
If anyone has used them before can you let me know what they are like, thanks.
I saw one yesterday in the Redhill Halfords.
They will ring up other stores for availability of parts, as long as you find the employee who can use a phone.
To me it looked like it would sit on the cubby hole - just. Ther may be a slight overhang, and it would be easily visable through the side windows.
It would need some hefty screws though! I wouldn't like to do an emergency stop with that behind my head.