I have had 'bonnie' for 19 years now, and often wished she had a neutral indicator. I have just stripped the engine totally (mainly just to fix numerous oil leaks) and bugger me, there is a switch on the inner gearbox cover (not connected to anything). It can only be a neutral indictor switch can't it? The wiring diagram don't show one though ??? (mine is the very first T140E, and has some 'E' and some 'V' bits)
76/78 T140 Bonnie, Suzuki B-King and Kawasaki ZZ-R1400.
The switch makes to earth when the gearchange quadrant is in the neutral position and the 'pip' depresses the plunger on the switch. Test your switch by checking that it flows electric with the switch depressed. The switch also makes fitting and indexing the inner gear cover a doddle - make sure the gearchange camplate is in the neutral position ( it should be after re-installing the gears) then offer up the inner cover with the 'pip' engaging the switch.Much easier than with the earlier covers.