The rapid flash is actually some sort of warning that not enough current is passing through the flasher or some kind of sensor. I had an issue before where i already replaced the bulb and it lit but still the rapid flash remained. Upon obervation later that night, it turned out that the bulb i replaced was actually a bit dimmer than the working bulb on other side. I cleaned the sockets and took out the working bulb and got another one using it as a sample and discarded the first replacement. After plugging it, the rapid flash was gone, either the socket cleaning or the new bulb replacement did it.Originally Posted by tuggnet