We are starting to get close to the same page now.
On all of my high quality home theater equipment, and most amps which have multiple inputs, there is a bias or level control. Some are hardware some are soft/firmware.
These need to be leveled so as you switch from one to another they provide the same dB level from the output. My wife's 04 Maxima has the same style of radio, and it does NOT change in volume from one mode to the other. She also can receive radio stations I cannot when we are parked next to each other.
This is what we want Nissan to review with R/F and fix. Find why reception is so poor, and adjust the amp to provide even output levels.