I don't have experience with your model but I've fitted loads of new stereos and amps to my cars over the years. These have always required a new live feed direct from the battery in the engine bay.
The best solution will be to follow an existing cable route through the bulkhead. There should be at least one visible from inside the engine bay, somewhere near the driver's side. Often near the steering rack.
You'll need to remove the trim from below the dash on the driver side.
Hope that's not too obvious
