Assuming that your 899 is an imaging unit (SI), then the standard imaging transducer that came with it for the transom is a compact transducer. The one that comes with the Helix 10, whether Mega or not, is a different one. You will still be able to use your shoot through and y cable for 2d though. Yes, there will be some work involved. I dont believe that the Helix 10 will fit in the dash without some type of mount, so it will not look like it came from the factory.
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