'Audible Warning'

 Just had my new (to me) Passat wired up and fitted with a towbar. Just as with my last car, the beeper is fitted in the boot and can only be heard with the engine on tickover and no music playing, Once on the move it's inaudible. Isn't it a legal requirement, not just to have it fitted, but also for it to be heard?

  On my previous, I had to get the installers to come out and fit an extension, looks like I'll have to do that again.  Why can't they check these things??? Or don't they have to???