wiring thru master switch?

Wanting to run new power supply wire to my new Lowrance bow sonar on 2004 P3. Previously had installed by splicing new power wire to factory wire at bow, but old wire is much thinner gage wire and Im suspecting intermittent problem with new depthfinder is at least partly due to weak power supply. Am planning to use old wire as a guide thru channel. Old power supply is controlled thru master acc power switch. Would rather avoid running new wire all the way back to battery, but wondering if factory power wire was originally connected directly to master power switch located near throttle arm on driver side? Or does master power switch have connection back to fuse block panel in transom area? Thanks in advance.