Three places:
Next to the starcom unit
Being adjacent or on top of the starcom will require no cable run.
In your pocket
requires a cable from the starcom location to your pocket. Very awkward. Being as how you have a cable from the starcom to your ears already.
On the bike elsewhere
(other than with the starcom) requires minor cable routing.
piggy back it with the starcom unit. Any water issues you think you’ll have with piggy backing it with the starcom unit, you already have with the starcom’s current location.