How do you replace the plastic lines?

[jbach36]

I can easily spot and replace dryrotted rubber vacuum lines on my car, but how do you replace the plastic vac lines (and some may be for stuff other than vacuum). They've become brittle and can break easily. My mechanic said they're hard to replace because they squirrel around and you have to find where they run to, and when you try to pull them out, they break.

I've looked at them in days gone by, and it's hard to follow where they go.

Any advice on this relatively simple repair?

1991 300d, 196k