Mac
Glad this came up, hopefully I will be at this stage soon.
KTB page 164 refers to the error in the schematic of the horns power supply being interupted by the brake light switch, and that the diagram shows a positive eartch, most Series D had a negative earth (mine upon disasembly was a positive earth, but I do not know if the wiring was original)
Vincent by Richardson, page 183 says the positive terminal is earthed after Frame No. RD12870 (mine is RD12776B)
JP Bickerstaff's, The Original Vincent page 118 refers to the same wiring diagram error as stated in KTB
My harness schematic from Rhode Island Wiring indicates a positve earth, with two seperate leads for the horn and two seprerate leads for stoplight switch, independent of each other, which on paper and in thoery seem correct. My reality maybe very different.
Dave