Chombi, you're one of the lucky ones (me, too - my Spinella is similarly equipped with an internal guide tube)! Unfortunately, a lot of builders simply drilled two holes, brazed a couple fancy (or not so fancy) fittings at either end and left it at that.

I had the devil of a time with my old Tommasini Super Prestige when I pulled the entire cable and housing out; probably took me 3 hours to get that thing back in

Good luck, OP, 'cause I think the only answer is trial, error and time...

DD