I was installing cables on an SLX Merckx and had trouble with the internal routing for the rear brake. The housing itself is too wide to fit into the frame. I'm not sure if it's supposed to be housed the whole way down or whether the entry point acts as a stop. But either way its too narrow for the housing.

I'm using a Jagwire cable kit. Are there narrower cable housings? Do I need a step down ferrule for it?

This is a new one for me.