Welcome to the Forum Ross,
That was mine. You have to intercept the circuit on one of the battery holders. I unsoldered one end, then added the switch between those points by soldering on some lead wires.
After talking with Marshal Gas on why they didn't put the switch in, they said the LED draws so little that the batteries would last over a year at continous flash, therefore it was unecessary. When I put a new indicator in our new 3400, I did not install the switch like we did in our old one. The batteries held up all year even while continously flashing in storage with the tanks off.
Unless you really have your heart set on doing it, the switch does not offer much savings in battteries and is a lot of work for little gain.