No picture available but I would think it would be windheild washer before coolant overflow but I could be wrong.
If it's for coolant overflow it should have a hose running to the fill neck on the radiator.
If it's windsheild it may have a electric pump hanging on it or mounted next to it (little bigger then a D size battery) or a hose running up to the windsheild washer motor if it's a GM platform.
1959 22' Caravanner
1988 R20 454 Suburban.