I just rec'd a 15' extension cable for my Eagle Fishmark 320
fishfinder. The instructions say that I must attach 12v power and
ground to the power/ground wires that branch off of the extension
cable's connector that plugs into the main unit. I was expecting that
I would be able to simply connect my existing cable (which has 12v
power and ground feeding it)into the extension cable and I'd be good to
go, but now with a cable that was 15' longer.

The latter setup above is was what I was hoping for as I would
sometimes like to simply extend the cable that currently comes out of
my dash so that I could locate the finder away from the dash when I am
trolling (I face towards the back of the boat when I troll; sitting in
a plastic patio chair which I put in between the two front facing
lounge seats, and in front of the middle part of the split in the
windshield that swings open so you can crawl out onto the closed bow of
my 17' I/0 runabout.)

Does anyone know why Eagle would not "extend" the power/ground that
comes into the original cable down through the extension cable, and
instead wants you to attach power/ground to the power/ground wires that
come off of the extension cable's main unit connector?

Thanks!