I had fast blinkers with headlights off or on. I looked here for the answers first, and didn't find them, so I'll post my solution.

Seems this is a common thing, and reading around suggested using a different part number than the original sockets.

I used STANDARD MOTOR PRODUCTS S829, Alternate/OEM Part Number(s): 12141302, 15306194 from RockAuto. They fit and lock in perfectly.

The dual element bulbs have staggered tabs on them, and if you manage to put the bulbs in backwards, you get the fast blink. If you mix up the wiring, you will get the fast blink as well. The blinker lights are the brighter element in the bulb.

If your lights only fast blink when headlights are on, check the bulbs & sockets mounted in the trunk lid.

There are 3 of these sockets per side. All 6 of mine were screwed up in some way.