It can be very frustrating and time consuming to route a cable internally through the frame, if you don't have the proper tools. I think Park Tool was the first to come out with a good internal routing tool, followed by many others.
Recently, I decided to get an internal routing tool for myself, as I needed to route new hydraulic hoses and Di2 wires through the new Cervelo Aspero frame and fork. Previously when I assembled the Canyon Endurace, I borrowed the Park Tool internal routing tool kit and it worked pretty well.
PRO Internal Routing Tool. It is made to be like a multi tool, instead of many separate parts. Comes with a storage pouch.
The left side has a big magnet for guiding the other adapters through the frame. Right side has the various adapters to fit different types of hose or outer casing.
There are three adapters, for Di2 wire, brake/shift outer casing, and hydraulic brake hose.
From top to bottom: Di2 wire adapter, brake/shift outer casing adapter, hydraulic hose adapter. Each of these adapters can be removed, to be attached to the other part of the internal routing tool.
Adapter that clips into the Di2 wire
This adapter is joined to a short stretch of cable that has a small magnet on the other end. This will allow the big magnet to guide these parts through the frame.
Alternatively, use the long wire that has a small magnet at the other end. First, guide the long wire through the frame using the big magnet. Then, stick the magnet that is connected to the Di2 wire to this long wire. Finally, pull out the long wire to also route the Di2 wire through the frame.
This tool works pretty well, but you need to know how to use it to maximize its effectiveness. What I like about this PRO tool is the addition of the Di2 adapter that can be used to guide the Di2 wire.
With this tool, I was able to guide the Di2 wires and hydraulic hoses through the Cervelo Aspero frame and fork easily. However, routing the Di2 wires and hydraulic hoses through the PRO Vibe Aero Superlight handlebar was a nightmare, as the internal space was very tight, with sharp bends. Also, the entry/exit holes on the handlebar were quite small which made it difficult to squeeze the magnet and cables through. That is no fault of this tool, just wanted to highlight that if there is insufficient space, even an internal routing tool will have limited use.