Battery draining fast

HAPPY NEW YEARS !!
I did get a maxi fuse type connector it's yellow . It makes sense to pull a fuse and not have to mess with the bulk head and those tiny screws that are hard to see and yet alone unscrew lol.
Well I did all that and the voltage is up to 11.97 - 12. Up and down noticed as I was working in the dark the dash lights don't work .
I finally got the engine wiring done ,now went into car under dash and looked at the fuse box and the fuse tabs are rusted and corroded . I'm going to tackle that today .
Can anyone tell me why there surface rust pattern going down the inside firewall just on the drivers side ? There's not carpet in it and it looks like its getting rain coming in directly over the bulk head . I need to fix this ASAP !
Thanks Bud27