On the right hand side of the boot as you're looking in on it. If you lift the carpet to see the spare wheel, on the underside of the carpet you will see a score line in the backing. If you gently run a Stanley knife along it, it will allow the carpet to fold around the changer. There's a bracket available from Porsche to mount the changer on, but if you have brackets on the changer already, use those.
The changer will mount to the chassis rail - four holes to be drilled to accomodate the brackets. The lead is best run around the front of the boot, down the left hand side as you're looking in and through the bulkhead just above the accelerator. If you remove the drivers carpet, you'll see a grommet and wires going through the firewall. Drill a 20mm hole in this location and run the cable through it, (remember to fit a grommet!). It's easier to drill it from the cabin side and it's also easier to feed the wire from this side, because you don't have to try and work on the three inch gap to the side of the fuel tank.
Regards