My 2001 C5 had the fast flashing turn signal indicator doing it thing telling me the lamp may be bad. I have learned how to change the lamp but this one gets weird. I pulled the lamp socket from the case and it started flashing. Nothing wrong with the lamp. Put it back in place and close everything and the next day it started flashing again.
I am starting to suspect a bad connection or lamp socket and have no idea how you change the wiring harness or how to remove it to be checked