turn signal wire harness ground wire Quick Q

[Jeff-n-Red]

68 c10
on the half-moon wire harness connector ( i already have the diagrams)

I just replaced my turn signal complete switch/harness (and flasher on fusebox previously), still no blinks
(wire tests good back through the truck- I fed 12v to the yellow and dark green on the harness and both taillights gave juice)

QUESTION: It seems strange that the the black wire on the harness connector (both ends of the connector) does NOT test as a ground with a test light - my dash, cluster panel, firewall, all test as good grounds

it's the black horn ground wire on the half moon harness
Should it test as a grounded wire- always? At least the connector blade that leads to under the dash?

Mine doesn't- if it should, WHERE exactly would be the next place that ground wire should go to (to troubleshoot).

I can't figure it out on the diagrams .

ALSO- Related? : the purple wire at the connector that feeds the volts to the switch seems weak with a test light- it lights the test light but not as strong as other 12vs spots- such as on the fuse box.
Thanks!