I've been working on an electrical project recently. It always involves threading wires through tiny gaps under tanks or under handlebars etc.
The handiest tool to do the job is something I've had in my shed (and used) for years.
I don't know where you can buy them- I haven't tried.
You hold the end as if it was a syringe and push the rod which extends 4 wire fingers. It's spring loaded so whatever you grab is held while you pull the tool through whatever tiny gap you needed to negotiate. It's about 600 mm long.
Too easy.
Blurry photo, sorry- tricky focus etc.