My left front blinker was out. I replaced the bulb, and cleaned up the corrosion. It worked for about a week.
Next step was to remove the blinker and clean all of the contacts. I put dielectric grease on everything... and it still didn't work.
Finally, I ordered a "used" blinker assembly. While installing it, I pulled the front headlight out of the bucket (I realized the connections terminate there) and found VERY corroded connections inside (red arrows in the pic). This was my problem - the entire time. Sadly, my original blinker assembly was fine.
Next time your blinker is on the fritz, and a quick bulb replacement won't work, check the connections inside the headlight bucket.
Good luck!