Sorry Phil, but the 3100L does NOT have a microphone - the L version is the headphone-only version of the 3100.
The wiring needed for GA use is shown within the User Manual linked by DB6.
Exactly what version/wiring you have currently is near impossible to figure out, particularly since the Telex website is very nearly the worst website on the planet.
It's a good bet the unit you have is wired for helicopter use.
Adaptors >
Headset Services Limited - Adaptors
(Disclaimer: I have no association or link with the above website... found it via Google)
- GY