I've researched all the threads I could find and need specific advice on what steps to take / what to replace first.

Some background:

Key fob battery has been replaced and it has been verified as transmitting a strong signal.

I have unsuccessfully tried to reprogram the key fob several times using all the technics mentioned online and in this forum.

Interior lights are working and the #9 fuse is intact.

When pressing key fob buttons, running lights do not flash, nor alarm siren chirp (I replaced the alarm siren several years ago with one that chirps.)

Air pump for door locks is functioning as the interior lock switch is working correctly.

Driver's door can be manually locked with a key, but it does not lock the passenger door during that process. (Or unlock it)

I have removed the panel where the audio amplifier is located and verified all electrical connections

I have checked the connections on the BCM (Body Control Module) but don't have the ability to test it.

Skreem allows the car to start without any issues.