From a quick Google search for people having similar issues, you might have a short in the wiring to ground on the secondary side of the relay (non-battery +12v side).
The relay energizes, causes the short, the short cause the drop in available voltage so the relay kicks out. Now that the relay has kicked out, there is power to energize it again, so it does and in doing so brings the short back into play starting the cycle all over again.
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Answers
The relay energizes, causes the short, the short cause the drop in available voltage so the relay kicks out. Now that the relay has kicked out, there is power to energize it again, so it does and in doing so brings the short back into play starting the cycle all over again.