Can anyone help identify what this round plug is for on the firewall behind the wiper motor?

Thanks,
Dave

http://www.chevelles.com/showroom/data/500/medium/Picture_0261.jpg

back up light harness to manual trans switch

back up light harness to manual trans switch

Thanks for the quick reply…now I just have to figure out what I need to purchase to make it all work… Looks like everything from the plug to the trans is missing.. Including the hardware and switch on the trans.... Should I look for anything inside the car or is that just the backup light harness.

If I remember correctly if the shifter was changed to aftermarket the reverse light switch did not work we put a manuel switch for the reverse light. It may even require the original shifter but not sure.