Need Help. New Boat Owner ..No power to any switches

Same problem..  Boat starts...master switch etc  nothing else works.......Went to batteries with meter and both read 12.64......One smaller red wire comes out of battery switch through small 2 post "breaker " and leaves as 12.64v...This red wire goes to the front console and comes out as 2 red wires. and 1 black.........nothing..Now red wires read  lower than 12v....  I took  long extension wire back to positive  post of battery and touched to wires going to relay switches and everything worked....So red wires must be corroded at some connection that( I can't get to)...Now my problem how do I get a new power  wire from the batteries to the front....any suggestions....

Thanks ..But I fixed problem by running new wire from battery to console ..This is my (93 yr. old) neighbors approx 2016 23 ft deck boat He bought it new when he was 90......It was sold to him with out a float switch on the bilge pump....He does not take it out a lot and it was on a lift  and covered..but was full of water ..(HALF WAY UP batteries)..NO float switch...I thought float switches were standard..DEALER told me it was an option...I believe this caused corrosion on the battery wire connection somewhere between battery and console....I told him to always turn on bilge manually each time he takes boat out....I bought his 2002 Hurricane and it only had 620 hrs....Float switches should not be an option...don't you agree

If it has one it failed..    Didn't last long......I will take a picture and get back....Thanks...