I want to provide mono (L+R) feeds into a helmet without stuffing the
amplifiers or affecting the loudspeaker output. So do I use a couple of
capacitors from the two lives or what? It's so long since I played with
this
sort of thing.

It would be easier to just wire up in stereo from the front speakers. Is
there a reason you can only have mono in the helmet? If so, just running off
the left would be easy, and don't play any Queen!

Christian.