Just to expand a little I fitted a power amp in the cubby hole on the driver's side of the boot (with first aid kit symbol on). I routed a power cable down the passenger side of the car from the battery through the rubber grommet in the cabin filter area and down under the plastic sill panel trims, through the spare wheel carrier and across the back of the boot under the plastic trim there. The preamp and speaker cables I routed down the drivers side of the car, again under the sill covers, to behind the head unit where I connected the front speaker feeds into the car loom.
There's a howto on this in the howto area.
One tip I discovered is that rather than fitting an extra fuse holder by the battery the positive distribution fusebox above the battery has a spare slot in it. Just get a maxi-blade fuse rated appropriately for your wiring, plug it in, and get an extra spade connector to fit the other side of the fuse and you're away. Battery connection made!
Might be worth checking you've got speakers in there beforehand but whilst the plod may have skimped on the radio I doubt they'd have had cars made without door speakers.
Kevin