No sound from drivers door speaker

[mmarksx19]

2007 3.0tdi with std stereo (not bose or b&o). Just one speaker in each front door.

Since I got the car, there has been no sound from the drivers door. Changing the balance and fader across the ranges I can get sound from each extreme except the driver door.

I removed the speaker and plugged it into the passenger door and it worked fine so I know it isn't the speaker itself.

So I think the options are;-

Wiring (nightmare to trace and check I assume)
Amplifier (any tests I can do to check for output on that channel?)
Or component protection (would it stop just one speaker?)

Any suggestions or things I can test?

Michael