AM audio fades after 10min of warmup. Replacement radio did not solve problem.

Your comment about the antenna is likely correct. This problem only happens after warmup. FM signals are higher frequency and are less subject to a problem with the antenna because the can more easily couple capacitatively. That is why I believe this is related to a possible warmup problem in electronic amplifier between the windshield antenna and the final antenna input to the audio amplifier. During warmup, if I hit bumps on the road, the sound cuts in and out which suggests a thermally sensitive connection in an antenna amplifier. After warmup, the AM sign cuts off entirely but the FM signal does not.

I'm not aware of any way to amplify amplitude modulation frequency... Contact > Crutchfields ... Audio specialist for any of your audio concerns.

There are frequently input amplifiers built into the low level, antenna signal input to audio amplifiers which input amplifiers amplify low level signals from the antenna prior to demodulation and further. It is possible that such an amplifier is located externally to the main dashboard audio amplifier and is located between that amplifier and the windshield antenna. If I could find an overall connection diagram for all of the components of the audio system for my car starting with the windshield antenna, that would be helpful.

Well it's obvious you be digging you some AM radio! .. Not sure if I even have a functioning AM tuner or not .. Crutchfields is best place to start. Good luck!

There is a local talk radio show that I listen to when driving. This is a weak station in our area. Otherwise, not much other AM content is of interest to me.

Yeah, sometimes AM got some different stuff that's for sure... One can find a ball game, talk shows and such, on AM that ya can't get on FM though sure enough... It's all good, we just picking at ya a bit... Them ol boys & gals at Crutchfields are pretty darn good at this stuff! .. Later. ▪️https://www.crutchfield.com/ISEO-rgbtcspd/learn/learningcenter/car/antennas.html▪️