Well I don't have a fenderside/stepside but I will GUESS, if that's alright. I assume there is no passageway from roll to inside of support.
1. Drill thru both walls of support into inside of roll. After fishing wire, patch weld or use 1/2" black plastic
plug/cap to cover visible hole in support. This is if I see the problem correctly.
2. A long drill bit might work angled thru roll opening.
3. You could drill thru both walls of roll and into support hollow, then patch weld up the visible hole.
Fishing the wire down in, is a piece of cake after you make access.
These ideas are a way of getting the conversation going into brainstorm mode.