Dome light

[chevyhed53]

OK, here's a weird one: I need to rewire the dome light in my '56 Chevy stepside. I don't want to wire it to the headlight switch, instead I'm just going to put a basic toggle switch on it. Problem is, I ran mechanic's wire as far as I could through the hole where the original wiring came through on the light fixture, and I can't make any progress--in other words, I can't figure out where to run the wire to power. I assumed it snaked down behind the driver's left shoulder (more or less) and under the running board and up into the firewall. But I can't do it. I still have my gas tank behind the seat. Any suggestions? Thanks!