The ongoing problem with my car audio - low sound volume on the audio player/radio.

This is a bit long - if you want to cut to the chase see bold paragraph by the %%% a few paras below.

No fewer than 5 specialist repairers told me the unit itself was faulty, not the speaker or other wiring. Some charged me for that privilege of wrong diagnosis. Some are based 50 miles from me and it cost me a lot to find out they are wrong.

The manufacturer has checked the unit (and charged me) and the problem is not the unit but the wiring from it. You'd have thought one of the 5 would've known that, particularly when I pointed out to them that the top of the dashboard appears to have been removed previously as the velcro strips on both sides are showing

So I am at square one with the problem, which is low speaker volume sound and occasional high pitched noises with or without the audio switched on if I go over a bump in the road on the right side of the car (OS loose wire?). When that terrible sound comes on sometimes it won't go away for ages and further distorts the already low sound in the speakers making the radio or CD sound incomprehensible. Pulling the fuse and replacing resets it as does sometimes a thump on the steering wheel or turning off the ignition etc.

%%% The manufacturer has suggested the low volume fault may be a loose wire or broken Scotchlok possibly as the result of a handsfree kit badly removed from the car by the previous owner. Where then are these kits typically wired in? Somewhere to the top of the dashboard or near the audio area in the middle where the radio is, any ideas? I apparently need to look for a loose wire touching the chassis or something, or perhaps a bad Scotchlok etc., something like that to find this wire that might be the root of the high pitched noise that sometimes comes on and the low volume problem that's always there. Something as simple as the speaker wire terminals or 2 wires fused and touching perhaps? How can I trace it?

Any tips - thank you in advance. I can't find a local technician that can be bothered, and the so-called recommended specialists locally all told me my radio was "knackered" and I need to spend £300 with them for a replacement exchange unit. (My unit is definitely not knackered or broken in any way, the manufacturer assures me, they've checked it twice.)

(Essex based)