Jack,
If you don't have a *G* switch in your L-Nav, then you must have an external
switch to do it. On a flapped sailplane, it can be a micro-switch in the flap
drive system to put the L-Nav in climb mode when positive flaps are commanded.
In a standard class ship, the switch is usually mounted on the stick.

Having said that, I no longer feel the need to switch. With GPS constantly
driving your position in the L-Nav, the distance to your selected destination
will always be correct. I just leave it in cruse mode with the audio off and my
back-up vario (B-40) on with audio up. I get audio only when I'm going up and
no distracting speed-to-fly audio, when crusing. If I feel the need to see what
old Mr. MacCready thinks about my speed, I just look at the speed-to-fly
needle.
JJ Sinclair