06 Corolla Base Model Antenna

[frafreg]

I replaced the factory AM/FM radio for a new one and I noticed that the AM reception is not nearly as good. Some stations are very low in volume compared the factory radio. The FM reception is fine. I installed a signal booster but that made the AM worse. I checked the antenna for short to ground with my meter and it read zero so I know it's not shorted. I removed the antenna and cleaned where it screws into the base but it didn't help.

What else can I do to fix this problem.
Any help would be much appreciated.
Than you in advance.
Frank