I have a 180 Freedom (Basically the same as the Horizon) with the same seating. I originally planned to put in six speakers. I ended up going with just 4, which eliminated the need for the Sub (many mount this on the starboard side under the built in cooler).
I swapped the old Sony head unit with the Sony
http://www.crutchfield.com/p_158M60UI/S ... ml?tp=1721 and the Polk 651 that everyone on the board likes.
I mounted the rear speakers just in front of the rear seats. The existing two were just a swap out (though needed some redrilling for the screw holes). The head unit was a direct swap out since I stuck with Sony. I bought the little connectors at Radio Shack so I could wire the additional two rear speakers into the factory wiring housing (think the whiteish clear plastic box). You break a few, so buy extra's so you can get it right.
I skipped the bow speakers (which would have been 5 and 6 and needing the amp). My wife was not happy, but there was just no good spot to mount them due to the wrap around cushion. The space was too narrow between the bottom of the back cushion and the seat part. I could have mounted them right in front of the helm on both sides, but felt someone sitting there would block the sound.
I have not found a picture on this PC, but will post one for you to see. Pretty easy install, just measure and triple check where you want the speaker as once you cut you cannot go back.
P.S. forgot to add, I put a remote stereo control on the transom so we could control the music while in the water.
viewtopic.php?f=3&t=629&st=0&sk=t&sd=a&start=120
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2010 Sea Hunt Triton 220 "Big Whiskey"2003 FW Freedom 180 Volvo Penta 4.3GL/SX-M "It'll Do Four Now" (Sold)
2012 Avalanche LTZ 4X4 