The only other factory option to go there was the roof switch for convertibles.
The bezel was the same from at least 72+ (possibly 70-71 as well). So even if the car had nothing there, the panel contains the hole in the back for the switch, there is just no opening cut in the front.
I imagine it was a popular spot to mount aftermarket equipment, just like the empty gauge spot on the standard cluster cars is often used to mount an aftermarket gauge.
It's definitely not for the electric washer pump in a stock config.
You could always trace the wires, as an add-on they should be pretty easy to trace down.