led tailight bulbs

I've seen that Grote flasher refuse to work in some applications. Grab the one I linked, which comes with a dingus that lets you swap the flasher polarity if it doesn't work at first.

Also, on a '68-up car make sure you're swapping the turn signal flasher (clipped to the back/side of the ash tray cavity in the dashboard) and not the 4-way hazard flasher (closer to the lower lip of the dashboard).