One of the many items you have to deal with while restoring bikes, is dealing with the sins of the previous owner’s efforts to ‘improve’ the electrical system. In fairness some of the ‘correct’ parts are difficult to find, but it also seems as if things electrical confuse and confound many folks to the point where after continued improvement and change, nothing works at all ! 
The wire harnesses of both the 1973 GT750 and the 1977 GT500 were in very poor shape – many wires had been cut, poorly spliced, the correct connectors were missing, the protective jacket had been removed or it had just weathered away with age. Replacement wire harnesses are available in some cases from Suzuki, and also from some speciality suppliers such as KnK Cycle but items like handlebar switch gear are generally only available via eBay, and of course these will be the same age and often the same state of repair as the ones you already have. I plan to rebuild the switchgear and repaint it as shown here, but the harness on the other hand is a different challenge. Luckily, I was directed to an outfit in the USA that supplies most of what I need !
Vintage Connections sells the OEM style fittings, both the 2.8mm latching and non-latching box connectors used in the headlamp shells for the harness interconnects to the instrument pods, and handlebar switchgear, as well as the larger 6.3mm box connectors (latching and also non-latching) used for connections to the regulator panel and ignition coils under the tank. They also sell the black vinyl tubing in sizes suitable for you to replace the protective sheathing leading from the switchgear to the headlamp shell, and from the headlamp shell around the headstock to under the tank, and the leads from the ignition to the electrical connections panel and under tank connections. I like their crimper, although for smaller wire sizes (22 ga. for example) I use a second crimper or sometimes actually solder the joint, just to ensure things won’t come apart at an inconvenient time !
Currently I have finished rebuilding the wire harness for the GT500, and I am ready to start redoing the switchgear sometime in the next few weeks. I’ve done a ‘hot’ test just to ensure the lights, turn indicators, brake switches and horn are all working correctly – so far, so good !