I doubt is has anything to do with the stereo. By the looks of it, it is probably as old as the car. I started alarm installs in 1996 and there was nothing like that piece back then. All the pagers that used the factory antenna had to plugs on the module, an input for the factory antenna and then the output back to the radio.
The antenna I speak of is more than likely for the reciever of the unit. It is very common to run the antenna up the a pillar and across the top of the headliner.
Best thing to do is get a test light on it and see what it does, it can only be a few things, lock/unlock, trunk pop, lights or a door trigger. By looking at the pic, i can't tell for sure but it looks lke it is spliced into the black wire. I would trace it back to the other end to make sure it is not for the radio or some other accessory.