Are you talking about cutting off the RCA jack and replacing it with a phono jack? I'd only ask why bother? I went to Radio Shack and bought a 3 foot cable with the RCA on one end and nothing on the other. Wired it as required and was done with it. (Well I was done till I ripped it all out to install this 120 like you did

. Now I'm doing it again.)
I think the RCA jack would be more water resistant than a phono anyway. Don't you?