Since I haven’t seen this side marker and turn signal light relationship addressed in the forum, I hope this saves someone some time and trouble. Here's my latest learning experience with my 2000 Sunfire Vert (original owner - 140K miles):

Problem: Rapid turn signal indicator on instrument cluster. Turn signal arrow on steady with lights turned on.
Probable Cause: Bulb burned out
Initial Exterior Examination: Turn on lights – Left front side marker light ONLY LIGHT OUT (note: side marker lights on 2000 Pontiac Sunfire GT Convertible blink with turn signal). Left front parking light / turn signal light on steady.
Solution: Replace side marker light bulb (194) … WRONG!!! Problem still exists.
Troubleshoot with multi-tester … appears that ground to side marker light is bad.
Accidental Discovery: Power on and try turn signal – side marker light flashes (YGBSM!). Front left turn signal on steady … does not flash.
REAL SOLUTION: Replace front left turn signal bulb (3057). (Note: Need to remove the headlight to access the turn signal and fog light bulbs)
Lesson Learned: Troubleshoot the problem (turn signal), not the part. Add to Initial Exterior Examination – TURN ON EMERGENCY FLASHERS!