You have to remove the climate control and the center console to access the amplifier. If you need specific instructions, you'll have to acquire a shop manual. One possible source is www.alldatadiy.com - but there are multiple sources you could try.
RepairPal Certified shops can diagnose and repair your 1997 Ford Expedition. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.