Proper way is to use meter to find which wire has power and one that doesn't if that switch is on (existing lighted switch). No power at any time will be ground. If using an ohm meter, and can't find a ground, might be necessary to make up a jumper wire and connect to battery ground.......if meter, when probing wires, with one meter lead to new jumper, reads continuity, that's your ground. Confused yet....?
Bob/Linda, 99 Dodge 2500 Cummins,2010 8280 Rockwood Ultra-lite 5er, PullRite Superglide-----
I'm retired......but work part time as a pain in the ass!........... USMC.