I found the problem about an hour after posting this thread. It was the main ground wire from the battery to the frame. I started checking grounds from the battery first and noticed that the frame had some pretty heavy surface rust where the ground wire was bolted to it. So I disconnected the ground and took a wire wheel to the frame until I saw shiny bare metal at the ground connection location. Bolted the ground wire back to the frame, and rechecked the voltage coming into the switch. The meter read 12.07 volts at both hot wires and 11.35 volts at the wire going to the gauge lights. Gauge lights are bright again!