Driver side speakers doesn't work.

[Brigand]

Driver side (left side of vehicle) speakers doesn't work at all. What could it be? What to check first.

PS. I am not much of an electric guy.

[edjack]

Possibly the amp. Common failure. Substitute a known good one.

[JimLev]

With you being in the European area we don't know what type of radio is in your car, possibly different than what we have over here on this side of the pond.

[Brigand]

I think it is a non DSP amp. The CD mode is a BMW professional CD. The Balance option is Balanced in the middle.

[Ed CT]

Possibly the amp. Common failure. Substitute a known good one.

This^
I had the same issue. Tested the speakers via their wiring and tested OK. Tried thru 2 different used amps - both BMW OE models - and the 2nd try fixed the issue.

You can also try cleaning the amp-speaker connections in the trunk