The amp is in the trunk near the spare. There is also an antenna amplifier.
Go to realoem.com and you can see the whole parts/part numbers.
If they diced and sliced the factory install you could be in a world of hurt trying to put it back. Make sure you investigate fully the extent of the modifications made to install the Sony unit as it can get expensive in a hurry and you could end up in the "I wish I never started this" category half way through.