I seem to have found how the dimming works, sort of... There's a menu option, but it doesn't dim enough.
And two disappointments.
The original radio would stay on as long as the key was in the hole. That must have been controlled by the DICE through the I-Bus. I've used all the original leads and the Kenwood will only work with the key fully on in the ignition position. Something to fix.
And the car relied on the display to show the clock. The Kenwood has a clock, but it's only displayed in radio mode. When the satnav is on, there's no clock on the dashboard.
But it works, and took me around the country in the last four days. Just a few quirks to find a way around.