Weird issue - Just started with Tach addition

[BillgreenC10]

1972 C10. Everything worked fine, I added a Tach from Bothers and everything still worked fine. The Tach was not working though, and I read that the tach itself needs a ground. Others say the tach is grounded and doesn’t need its own ground wire, even though there is a grounding bolt on the back of it.

I didn’t ground it, but everything was still working so I decided I would get to it when I could. It’s been about 4 months.

Just recently, if I turn on the parking lights or headlights, the turn signal indicators on the dash light up. If I activate the left turn signal, the right indicator on the dash flashes, and Vice versa. The turn signals themselves flash correctly based on left or right turn.

Could the lack of the tach ground be it, even though it’s Beem fine for 4 months? I’m gonna add the tach ground tomorrow, but while it’s torn apart I wanted to see if you guys had other advice of things for me to check?

I suck at electrical, so feel free to talk to me like a 1st grader. Thanks for any help.