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I’m not going to be able to do that in this frame unfortunately.
Are you sure?
the frame is old and has no internal routing for wires
You're only routing a cable out from the seat tube, then down along the trad mechanial cable path.
What frame is it? You should easily be able to drill a small exit hole out the bottom of the BB, assuming there isn't one. Pretty sure I've drilled a water escape hole in every steel frame I've had that didn't already have one.
Inside the seatpost, then run the cables to the mechs and bring them out there. Let me dig out a photo of mine.
See it here:
https://www.lfgss.com/comments/17233078/