If he was in a company car, that matters. The company can maybe call and activate OnStar and locate the vehicle on the say-so of the company account holder? His CEO? I'm not sure if that's possible physically, but it seems like it should work legally. That's just off the top of my head.
Also I agree with getting the cops involved all the way. Get that GF to press hard on them to come up with the proper subpoenas or whatever they need to unlock things at OnStar, Apple, Samsung, or whoever. If she can't or won't, do it yourself.
On a more personal, animal, psychological note..... that GF may have more answers than she is willing to part with. She may seem all up-and-up to you, but relationships have a way of going really sour. I don't know you, I don't know him, I don't know her. I'm just saying. It had to be said. Homegirl may just be evil as they come. It's not impossible. Look, I don't know you but I know people. They are mammals. They are animals. They can do anything. Don't necessarily go after the GF, but don't just rule her out either.
Other than that, go physical. Go objective. Go pragmatic. Go real-politik. Do actual organized pattern searches of the roads and the ground around them. Be smart about it. Do as Palum and Iannis say (they are both smart as hell). Work smart not hard. Nature does not reward emotivism, tears, or displays of virtue. She rewards hard thinking and real work. Get into the mud and the muck and try to find him. Use math and logic and search the most likely places first.
But that is secondary.
I would use the objective tools known as OnStar, Samsung, Police, etc first.
Just my 2cp. Take it with a click of Salty.