Where is the stock radio amp for a Ford Windstar premium sound have separate and if so where it?

2001-2003:

Premium Audio Upgrade: Amplifier under the driver’s seat (rear left).

Regular and 6-Disc CD Audio: No amplifier.

Early 2003-2005:

Premium Audio Upgrade: Amplifier in driver’s footwell under carpeting on the left wall (rear left).

Regular and 6-Disc CD Audio: No amplifier.