I got this NOS Helphos windshield washer hose from Charlie Harlock a while back:
This is the accessory hose that draws air from the spare tire to keep the windshield washer reservoir pressurized. There's a check valve that ensures the spare's pressure doesn't get too low. I believe a similar part eventually became standard equipment on later Type 3s.
The VW 1500 version is considerably longer than the one for the 1200. Here's the instruction sheet:
[Insert "clean windshield at the expense of an inflated spare" joke here.]