The adjustment I'm referring to is for the switch at the rear brake pedal. You can adjust the switch position up or down using the 2 plastic hex nuts on opposite sides of its mounting bracket. If the large red brake light in the rear is working, and the switch is properly adjusted , this light should come on with a little less than 1/2 inch of pedal movement downward from its resting position.
If the side yellow markers do not flash a few times when the brake is first applied its most likely to be a bad connection at the plug on the flasher/ beeper unit. You can unplug it and look inside for a misaligned prong. I put dielectric grease in these and all the other connectors to maintain there conductive integrity.
Are both the front and back side markers affected?
Let us know if this solves the problem, thanks. Buyingchina