The #7 Checkball (Reverse/Reverse Clutch)
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The #7 checkball is located in the valve body. The checkball is seated against the spacer plate in order to force reverse fluid through orifice #12 before the fluid enters the reverse clutch circuit.