Ah yes, the dear old Lucas bullet connector. Usually seen as a bright green crusty lump with wires coming out each end.
In the industry when doing development work on engine management systems it was always necessary to have the electrical system exactly correct (thinking when we used current production cars or "mules" for future powertrain development). One main principle is to have a common "earthing" point where all critical negative wires will come together at one point to ensure a consistent ground voltage. You never rely on bodywork to return current to "ground" like in the good old days, far too unreliable.