When I used to run lights with wires –
Helmet light – Wire ran straight out of the light unit, loosely velcro fastened to the rear of the helmet, battery and excess wire in the back pack. On the rare occasion I rode without a pack, it just went down my back inside the jacket and into the rear pocket (I made a little hole for the wire) where the battery was.
Bars light – Had a neoprene type battery holder that had velcro straps, so straight onto the bars with any excess wire tucked into the neoprene holder. it came with the light unit from Troutie. Also seen a small camera case used and velcro’d to the bars with all excess wire tucked inside.