Mk1 Focus - Right Side Speaker Issues

Hello all.

After going through multiple issues with my MK1 Focus, I believe that am down to the last issue (touch wood).

I have been having an odd intermittent issue with the speakers down the right side of the car (mainly the front).

At random points, the speakers on the right side will cut out. Usually they will not come back until the car has been completely powered down and left a while.

I have noted that sometimes the rear speaker stays on when the front cuts out, but it is usually both.

Although the cut outs do happen sporadically whilst driving (even slowly in a straight line), I have noticed them cut out under the following conditions:

• When opening the driver side window
• When shutting the door (if music was playing when it was open)
• When taking a sharp turn

The speakers very regularly do not come to life when I start the car at all,

I have found that, if the speakers do not work from start, sharply raising the volume has kicked them in. This has never worked if the speakers have stopped during use.

I have so far:

• Removed the speaker as checked the connections. No obvious corrosion was noticed.
• Tested another speaker. I had a speaker spare from a replacement door. I tested this in place, and it suffered the same symptoms, which in my mind rules out the speaker itself.

Can anyone suggest what may be happening here? The fact that it often affects the rear speaker also make the issue even stranger!

My next test will be to swap out the stereo itself, but I can't imagine that being at fault....

Although it isn't exactly the end of the world, this really does my head in when on a 4hr drive!

Thanks

Simon