it can be that your reading through a complete circuit, that is grounded on the back end. Are all the accessories, light switches, everything off. Something as simple as your domelights could potentially rad through to ground, as with your door open, the switch completes the lights path to ground. Only way to avoid, or figure out where is pull out every fuse, and continue to test continuity to ground while putting each fuse back in until it returns, then you will know what circuit your reading back to ground on. Then remove that fuse, and continue to see if you get that result with the rest of the fuses.
68 Mustang FB, Eleanor body kit, pepper gray, TCI full chassis frame cradle, 9 inch w 3 link, torque arm, panhard bar, coil overs, wildwood brakes. 427 dart CHI 3v heads, intake