I have an older solid state scanning marine VHF radio I'm trying to get
going again, after years of non-use.

The mike is missing. These radios all used a simple PTT mike---a common
ground, 12+ to one side of the PTT switch, and the audio from one side of
the mike element on the other. Three wires total. I'd like to put a plug
on a CB mike I have and use that for the marine VHF. I have no manual or
schematic, and little chance of finding one (I'm searching).

The chassis mike socket has 5 pins. I can easily find the ground pin. By
powering the set up, I can then find the 12v+ pin. That takes care of two
of my three wires in the mike cable.

Question being, then, how do I determine which of my remaining 3 pins is for
the audio lead?