I did the same thing with my 68dart when I first got it

also drove me nuts looking into why the driver side brake light did not work

pass side goes thru it but did not work,worked with the pass side blinker blinking though

come to be the turn sig switch on the colum,handle hung down from age and would cancil the brake light but not blink yet

then I looked all over to figure out how the rev light worked...last place i would have thought to look was down on the lower part of the colum under the dash

and 68 was the only kool any kind of mopar with the bullseye side marker lights

have you went to the newer style solid state elect voltage regulator yet with the 1 wire 68 alt?

that was one of the best things I did to mine,that and pertronix points conversion in the dist