One channel keeps cutting out until I crank up volume - help

[HiFiHunter]

I've a Vauxhall Zafira A, with a Kenwood KDC 7044 head unit, and mix of Kenwood speakers, crossover and Hertz tweeters (bit of mix due to fitting woes) at front. Rears are all stock Vx and all stock wiring. No separate amp or sub in use. Had the lot fitted several years ago and all had been working fine, until a few months ago.

The front left channel has a habit of cutting out. I can move the balance over just to the left, and get nothing. But if I crank the volume up a lot, it'll cut back in. I can then turn the volume down again and it'll keep working for a while. Also going over a heavy bump will often trigger it to cut in or out. Right front channel is rock solid, always works.

So, where should I start my diagnosis?
A loose connection somewhere? Big multi plug at back of head unit looks ok at casual observation.
Dodgy speakers?
Head unit on the way out?
Any ideas what is the most likely culprit? Best/easiest approaches to isolating the issue?

I've always been meaning to upgrade to Hertz speakers all round for consistency. Wondering if now is a good excuse to do this.

[trickywoo]

I'd check for a loose connection at the speaker first and then the speaker itself before moving to the head unit.

[defblade]

Loose or more likely lightly corroded connection somewhere between the HU and the speaker. If it's the whole channel drops, then more likely either at HU or Xover, if it's just mid or tweet then between Xover and speaker. Could also be an internal problem in the Xover - try swapping them left to right leaving everything else alone and see if the problem swaps too.