Simple. Put a ring lug that will fit, on the blinker mounting bolt under the nut, on the end of a wire and run it to the "new" battery negative. This will electrically connect the two battery negatives together thru the frame but that isn't a problem. Only the loads tied to the "new" battery positive will be powered from it. Your battery charging circuit, if you have one, will only charge the "old" battery since the "new" battery isn't tied into the charging circuit on the positive side.
Either that or just connect the "new" battery negative to the same place that the "old" battery negative is connected (which should have the frame grounded anyway).
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