Easiest tap for electric gauges - 1974 duster

Hi guys - I got out my test light and multimeter in the 20v range, and found a hot-on-ignition&running wire under steering column. Tapped it to power electric gauges.

Working fine, until I turn on turn signals: the gauges blink with the turn signal!!

What is the simplest/easiest wire to tap for these gauges? They are mounted under the dealer ac vent, I'm about to tap my aftermarket radio yellow wire, at least I know that would work, but I'm curious if there is a better option?

Thanks guys!
-Ryan