I would ask what you intend to use the port for. If you have a self-powered OBD reader, you really need just the OBD bus wire (pin 2) plus ground (pins 4+5). If you also need +12V, a third pin #16 is needed. All of these inputs are available at many places in the vehicle and can be spliced or teed in.
The passenger foot well is the location of the "star" connector where all the OBD devices junction together. Every computer in the C5 is on the OBD bus. You can tee-in anywhere an OBD wire is found because it's a shared one-wire bus. A couple other possibilities are under the rear toneau cover with the RFA (TPMS/keyfob receiver), or in the trunk with the F45 controller (if so equipped).
Wikipedia.org OBD diagram
If you need 12V with significant current, make sure your power draw will not blow a fuse for the circuit you tap into.