Electric windows half working

[MadMax]

Three weeks ago I bought a 1986 80 GL. When I bought it there was a window switch missing from the back door, driver's side. Sprig has sent me one from a pre-facelift B2, which I fitted yesterday. The switch looks different, but it's the same size and same connectors and went in fine.

All four windows now work, but the back two only work on the back switches (the ones in the back doors), the front "rear" switches (i.e the ones on the dash that operate the rear windows) don't do anything. Is there an obvious cause for this, before I break out the Fluke?