Ok, the fog is clearing, when the OP used the term *header panel* the rad header IS the header panel. The headlamp mounting panel is all bolt-on plastic.
IIRC there was a BOSE stereo option on some of those GTPs, that might be a connector for an option your car doesnt have. Snag a set of the factory service manuals and it`ll have all that info in there.
How many pins total, and what wire colors? At that angle, I see only 8 pins.
Does the car have an aftermarket stereo? If so, my total shot-in-the-dark guess would be maybe it's for a factory amp that was bypassed.