Use an electronic flasher for LED lamps
I have replaced every incandescent lamp (except the headlamps) in my 1995 Buick Regal with LED lamps. The flasher would not flash for a turn signal, but the reason was that the original flasher uses a thermal element that depends on the total lamp current to heat up and open the contact, then after a half second of cooling, it would close, and start a new flash cycle. And if one bulb burn open, the flasher would not operate.
My solution was to buy an electronic flasher that does not depend on the lamp current at all. It uses a small electronic timer "IC" (similar to the NE 555 for those who know) and a power transistor. So it would operate if all the original bulbs were incandescent, LED, or a mixture. The only weak point (which is not that bad) is if a bulb failed, the flasher will continue to operate, so the vehicle owner must test the turn signals often and verify that the lamps (incandescent or LED) are functional.
The electronic flasher is sold at major auto supplies. Mine was bought at Western Auto, now known as Advanced Auto Parts.
No Volt or Bolt EV yet!
2009 Chevrolet Equinox 2LT
Level 2 JuiceBox EVSE installed at 7.2 kW
Registered EVSE at PlugShare
Waiting for the first EV to charge!