The dealer antenna should work just fine.
Is the antenna making good contact to the vehicle body as a ground? Does the antenna lead wire connect properly into the back of the radio?
Is the antenna wire between the radio and the physical antenna old? It may have a wire break inside of it and need replacing.
How is the antenna seal to the body? Is it damaged or dried out?
Now for the other questions.
Before the antenna problem, did any of these things take place:
- Installed new plugs and wires (non-resistor type)
- Install new coil
- Added other electrical accesories to the vehicle, ie radar detector, big sound AMP, power inverter, ...
Sometimes devices or componets can cause interference with the antenna from working correctly.
Now for the radio.
Was this radio new or the original in the vehicle?
If it was new, it's possible that the wire lead doesn't make good contact to the radio. So, wraping the wire lead center tip with aluminum foil could help. Just make sure the faoil doesn't touch the ground on the wire lead nor the radio.
If its the original radio, diodes, resistors and the like could have gone bad making the radio reception lost.
Like Alloro mentioned. A good bone yard would help for either an OEM radio and antenna. But since you already have the dealer antenna, chances are it's fine.