Issue with turn signals in 70 Dart

This is troublesome. Might STILL be a bad flasher but make sure you replaced the correct flasher. Your car has two, one for hazard, and one for turn. Maybe you simply replaced the wrong one?

How to tell? Easy. Hazards work? Pull the flasher. If they quit, that is the 4x flasher. You can swap it into the other flasher socket to see if that fixes the signal problem

Other than that, ???? Time to pull the harness connector off the signal switch. Use clip leads to feed power into each 'bulb' wire, that is, feed power into left rear wire, see if the left rear and ONLY the left rear lights nice and bright, etc.

THERE IS a possibility this could be an effed up bulb. I'd just pull all 4, clean, inspect. Sometimes, bulbs "partially" burn out and weld themselves together internally. This can make interesting troubles.