Does the aircraft have 1 Navigation radio or 2?
If 2, do both of them have the same issue or just 1?
This is on an ILS or LOC approach, (GTN in NAV) not a GPS overlay approach, correct?
If the answers are 2, both and yes, I would go towards the antenna, coax, antenna combiner (if installed, typically at the antennas), antennas splitter (if dual navs).
If the answers are 2, no and yes, or 1, N/A and yes, a bad nav first, then most of the above.