Allen,
Are there indications that the windshield washer bag was mounted on the left front fender apron near the top between the radiator support and LH shock/spring housing? Two holes were required. There should also be 3 holes for mounting the windshield washer pump (two speed) below the area where the bag is supposed to be mounted.
If those items are not there, I would suspect that the rubber plug was to installed on the firewall to fill the hole where the pump wiring would have gone but punched inadvertently.
One more thing. If that is a Ford rubber, it would have a Ford hardware part number (6 digit number, a dash and the letter "S") on the outer ring.
Jim