I didn't notice the vehicle it was installed in from your post ?
Anyhow, you probably have a diversity antenna like I do in my Toyota 4Runner.
Instead of one antenna jack from factory radio harness, there are two! One normal sized one, then a smaller sized one, plus a third wire.
If you have a diversity antenna system, them you need to crimp off the smaller connector don't use it, and only connect the larger one.
This is probably what your installer did.
You need to take the third wire from the three wires from antenna wiring cluster, cut it and send it 12v Power.
I made sure to tell my tech about this issue for diversity antennas, and sure enough when he came to the antenna wiring from the truck, their where three wires there indeed like I researched from the Toyota forums.
I told him about this mod, he did what I said, and I seem to be able to tune am and fm stations fine, so he must have done the mod right, otherwise I would have very poor reception like you are experiencing.
hope this helped.