You'll need an amp thats suited to powering the sub, too. You could also do with finding out what gauge wire was used to run the cables to the boot, I guess the previous owner might know but you could just measure the diameter of the wire itself. And make sure that the amp wire is fused in the engine bay - wouldnt want one without a fuse! eBay is a good source for cheaper amps ;-)
And MG-JOCKEY, next time you're going down to York, try out the A697, its a nice road and the ZS will love it:
road review clicky.