'71 Challenger convt. ( base ) was 318-2, 904 auto, 7 1/4 rear. Now 440-6pak, 727 auto, 8 3/4 rear. Got the car out this spring & fuel gauge, B/U lites & LH T/S all had issues over the non-driving season here in Mi. ( No, not mice chewing issues, not in MY garage ! ) Gas gauge issue was resolved by installing new fuel sending unit ( plastic float developed a leak & didn't float anymore New one is BRASS . B/U lite malfunction was caused by molded rubber plug issue on trans switch.....replaced & now working. LH T/S lite would light but not flash; RH flashed as usual, as did 4-way flashers. Cleaned-off ground wires, bolt & captive nut on rad support for battery/engine/light harness ground & put Vasoline on for corrosion protection. Monkey-farted around with LH T/S housing....tapped bottom frt. valance screw holes in housing deeper & installed slightly longer screws. LH lites now flash, but issue now is the opposite side lites will flash ( faintly ) while T/S is activated.....doesn't matter which T/S is activated, the opposite will flash; including the headlites....brights & dims. Engine wiring harness has been replaced with repop to hook-up idle solenoid for center carb & change to electronic ( Mopar ) ignition. Headlite harness, tail lite harness, & under dash harness are OEM 1971 vintage. Turn signal & ignition switches have both been replaced, all light bulbs are NEW. Local garage is stumped trying to figure what's wrong; says voltage drops are all within spec. Tried new T/S switch with same results. Ideas on fixing the problem ?? Everyone says it's a ground issue. What am I missing here ?? Help !!