Since the car was not prewired for phone, it was built prior to 2002 (therefore you have the old round pin radio) and you do not have DSP my best guess is that the door speakers are not the dual voice coil speakers.
I bet once the door panels are off you will see loose speaker wire for the telephone system not wired to anything.

two options.

1) upgrade from round pin radio to BM54 radio in order for telephone to play via all speakers like the 2002 and up

2) change the door speaker on the drivers side to dual voice coil, or install another speaker in the door on the telephone audio wires.
This is not a common occurance as ALL US BMWS are prewired for phone, and only the X5 before 2002 had that old radio which without DSP used dual voice coil speakers. one voice coil for audio and the other for phone.

Martin