Dim/Bright/Dim/Bright/Dim/Bright, about 1 to3 seconds of each?
That indicates a self-resetting thermal circuit breaker in either the converter or battery line attempting to charge batteries with a shorted cell or cells
I woudl recommend you have your batteries tested (Simple voltmteter test may be enough, you can do that yourself, a LOAD test is better) and if they fail replace them.
Also, while you are at it, check all the usual suspects (Connections, 'epcially chassis grounds)
NOTE that it could also be a short "elsewhere" than the battery.. However I'm going play the odds and suggest you test those cells.