Depends which air-air missile.
In the F3, there were missile management panels in both cockpits. Generally the backseater played with the radar missiles and the front seater the heat seekers, but either could do either.
Either type of missile could be slaved to the radar (so effectively the nav controlled where it was looking), or could be controlled in various HUD modes by the pilot. I say HUD, some of the modes went outside the HUD FOV.
There was also a mode which would allow the pilot to manually steer the heat seeker onto the radar angles given by the nav, which was very handy when chasing down Jaguars, as it was hard to see the little buggers down on the deck in scoshy wx. Also handy, especially with Buccs, not having to descend to quite their height, which might be 30 foot
Late Arm was front seat.
Trigger was front seat.
I believe the F-14 allowed the RIO to fire the radar missiles, which is the source of some of the confusion.