This is a variation of a VOX (voice activated switch) circuit.
An example can be found at
http://www.rason.org/Projects/basicvox/basicvox.htm

This example is designed to be triggered by a microphone, not high
powered speaker outputs, so the resister values will need to be tweeked
on the input side. (Mike Input). The output of the circui is labeled
PTT in the diagram. This would be connected to the coil of a relay that
switches the the signal to the speakers. The other side of the relay
coil would be connected to Vcc (+12v). You'll probably want to use a
double throw relay and shunt the AM/FM radio output through a pair of 8
ohm high-wattage resisters as some amps don't take kindly to open circuits.

These must be marketed somewhere as finished products, but I've no idea
where. I bought one many years ago at a Radio Shack (I think) to switch
between a CB and an 8-track player/radio (you old farts out there will
know what I'm talking about - for the benefit of you youngin's, an
8-track was a predecessor of the cassette tape).

Steve wrote:

I have a pair of speakers in the cockpit coaming for the AM/FM radio..

I would like to figure out a way to connect the VHF external speaker to one
of these, so I can hear any calls on it in scan or monitoring of ch16.

Both radios are below deck so I can only hear them in the cockpit on a
remote speaker..

I know I could just use a toggle switch, but would prefer to find a way to
combine both outputs so the VHF would be heard over the AM/FM output.

Is there a simple way to do this and prevent one unit from harming the
other??

Steve
s/v Good Intentions






--
Dan Best - (707) 431-1662, Healdsburg, CA 95448
B-2/75 1977-1979
Tayana 37 #192, "Tricia Jean" http://rangerbest.home.comcast.net/TriciaJean.JPG