Dual Battery (Diesel)
WARNING: This page is about a different car, the 2002 Ford RV Cutaway, 2002 Ford Econoline, 2002 Ford E550 Super Duty, 2002 Ford E450 Super Duty, and 2002 Ford Cutaway. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect secondary battery positive battery cable from primary battery positive terminal. Insulate secondary positive battery cable with non-conductive material. Disconnect primary battery negative battery cable.