My absolute first reaction would be the chime module!! It does have a constant battery draw and I would think that to blame before the Nav. I have never had a radio draw enough current to affect a battery. Interface modules, on the other hand, have been a constant source of problems for us even the good ones.

Well....what if I disconnect the interface from the constant power source and connect it to the power source that is given when the ACC is on. Would that work for the interface? You only need the interface when the car is running rite? I will do some troubleshooting myself and see what I can come up with.

I would elininate the interface altogether for now at least and see what happens. I agree that it would appear as though the interface is keeping onstar powered up constantly.