Grounding a power feed momentarily can only result in making your test light light up.
Turn on the signal that the indicator works and then find the pin that makes your test light flash.
Then turn the other signal on and test the pins on both sides of the one that flashed.
One of those should also make the test light flash.
If it doesn't then the problem is not in the cluster, but in the feed to it from the signal wiring.
