Paki-describe na lang yung OEM antenna. Is it telescopic or is it a s/s whip or rod made from fiberglass? Is it vertical or can it be adjusted to tilt? O baka windshield mounted made from copper flashing? Daming tanong

With more info, I hope we can give you the right answer/s on how to check the antenna system. Of course, there is that possibility that there's something wrong with the receiver's RF preamp.