Brian,
I looked at the 69 Electrical Assembly Manual and Ford Car Parts (MPC), and it appears that 69 and 70 back-up lights are unique to those years, and not carried over from 67-68. No differences are indicated but who knows what the factory did.
From 69 Electrical - C9ZB-15500-B, RH, C9ZB-15501-B, LH, as assemblies.
From MPC - the assembly above are made up of 15110 (gasket), 15511 (housing) and 15514 (lens). I assume the wiring is part of the housing (subassembly). For 69-70, a full assembly and the lens were serviced. For 67-68, no fully assembled light was serviced, just parts.
You can down load a copy of the MPC here -
http://squarebirds.org/Manuals/1965/1965-72FordPartsTextCatalog/Jim