Glad you've got isofix 🙂
I've never seen a top tether for a c5. It's definitely not listed in any of the accessory brochures I have which include 1997 (for 98 model year), 2001 and 2002.
However if it does exist, and assuming you have an Avant I'd guess the tether would be attached to a bar that would slide clip behind the seat backs where the dog guard would go.bif that makes sense.