Vito W638 clocks malfunctioning...as usual

My 2001 W638 Vito has had issues with the instrument panel for as long as I have owned it. I know that this is a very common problem on this generation of Vitos, so rather than buying a scrap part (probably just as faulty) from eBay, I sent the whole panel away to ACtronics (an auto electrical specialist here in the UK) for a complete rebuild. When it came back, I refitted it to the van...and there was no difference. I rang ACtronics, they said return it, they checked it, and it came back with a clean bill of health. Still no change.

Here are its particular idiosyncrasies:

The needle gauges will frequently die while I am driving: the speedo and tacho freeze where they are, and the fuel and temp gauges fall to below zero. Generally, they'll stay there, and won't come back to life until the engine is stopped, left for a while, and then restarted. Of course, now they are sitting at 60mph and 2,500rpm or whatever, so will give utterly ridiculous readings. They will reset if the ignition is turned off and on several times: turn the ignition on, wait for the warning lights to illuminate, turn it off again, and the needles will drop roughly 20mph and 1,500 rpm respectively. Do this until they are at zero, and you are good to go. Until, of course, the whole thing expires again five minutes later. While the needle gauges are in this state of death, everything else - all warning lights and most of the illumination (more on that later) still function as normal.

So far, so typically Vito. But there are a couple of strange things that I have noticed:

After I park up (invariably with dead gauges) and turn the engine off, no amount of coaxing (via the turning-the-ignition-on-and-off trick) will tempt the clocks back to life, until I open the driver's door. I'm not imagining it; as I open the door, the fuel and temp gauges come back to life, jumping back up to zero. I assume that the contact switch for the interior light makes its connection, and somehow (bad earth?) this energises the instrument panel. It doesn't matter whether the interior courtesy light is switched to "off", and therefore not part of the circuit: the effect is the same. Also, you can switch the interior light on and off as much as you like without opening the door, with no effect. The door has to be opened.

And the other thing: the illumination for the fuel and temp gauges has never worked, not when I first bought the van, not after ACtronics' first rebuild, and not after their subsequent check, in which they specifically addressed this issue, amongst others.

I assume that there's some problem with the supply to the instrument panel, then. Is there anyone out there with any ideas or suggestions, or a similar experience? Does anyone have a wiring schematic for the loom that supplies the instrument panel? Why do all the warning lights always function as they should, but the needle gauges are intermittent? And why on earth should the fuel and temp gauge illumination not work in the van, yet tested ok at ACtronics?

Thanks in advance for any help, suggestions, sympathy, commiseration, practical advice, feedback, jokes, abuse, whatever..!