I find it's best to trim new cable to length and solder before ever inserting it through the housing. Once it picks up any grease or oil of any kind, it's impossible to get solder to stick. Although I did manage to clean the used ends once really well with acetone in an ultrasonic cleaner. Then dipped in battery-acid to etch for a minute or two. Then the solder stuck, but it was a lot of work to get a clean cable-end...

BTW - the plumbing-solder has way too much lead to stick well. You'd want an electronics-grade solder made primarily with tin & silver.