Still faffing with the radio; which isn't audible through the speakers over about 40-50mph.
The head-unit has separate amplifiers on +ve and -ve speaker lines & with headphones taking a common ground it needs a little workaround. Connecting left and right -ve or +ve lines will upset the head-unit amps.
The solution, cribbed from googling and reading through various forums is a current limiting and AC passthrough circuit wired into the +ve side of the speaker feeds and the common ground from the headphones back through the chassis ground:
I'm also incorporating a simple double pole single throw switch which isolates the speakers when not in use. I can't use the normal automatic switching 3.5mm jack socket due to the requirements of the circuit.
Decided on a small aluminium bezel for the components:
...and in situ:
Works well enough while stationary; test will be whether its any better when moving and through speakers added inside my helmet.