voltage issues with Nav lights on

Ok, this is something I've had as an issue since I owned the boat, but can not seem to find a solution. When we are just cruising at dusk, and turn on the navigation lights and anchor light, my voltage goes way down to 10-11 and the volt warning light comes on. If I turn the nav lights off, it's back to normal. I even disconnected the bow light and turned them on, same problem. Any guess at what i can do to eliminate this problem? Do i need to run new wires to that light?
Thanks
dan

Thanks Pete! I appreciate it!
Dan

Seb,
Yes, the main feed to the ignition switch. From there also check out the wiring to the breakers too. Don't forget the ground wire from the engine up to the dash as well.

This sounds like a job. i need a trained gopher to run the wires, lol.

Hey man I have the same exact problem in my 87. It has to be somewhere behind the dash or a ground / wire issue because I connected an extra volt guage up to the battery this past weekend and at the battery itself the volts stayed right where they should but the guage on the dash dropped wayyy down with the running lights on, or any accessory for that matter. I havent dug in to find the problem though.