BLK. IS NOT A GROUND
I'm going the other way (putting on fairing)But have aftermarket lights now(I put on).
!st check yo bulb (in housing,Lens off)with derect to batt.The smaller eliment is running light,larger is the winker,keep track.Most likely yo housing is plastic why you have 3rd wire(grnd.).
O.K. the solid color wires are the winker,Stripe running lights,Grn are grounds,Blk. are used for starting purposses,(can be # of things)Mine "77"F2 ,one to clutch switch & one to neutral switch,
NOW,My Brn. wire is not connected(accually just hanging there)It is live when using brake's,
Thinkig of having a idiot brake light (using it).
Will be on sight off N on all day(info) doing my build.
http://honda-cb750-s.456789.n3.nabble.com/piute-1977-CB-750-F2-SADDLE-BAGS-FAIRING-TRAILER-HITCH-touring-td3194402.html
1977 CB750 F2 Super Sport
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Native American from central Cal, Kickstand UP in S.W.Missouri,