This is really confusing me now.
I put TomTom back on its cradle for the journey home last night. When I first set out, the car decided that the lights needed to be on. A few hundred yards up the road, it changed its mind and the lights went out. TomTom (somewhat predictably) shut down the moment this happened.
Then the lights stayed off for a while, but TomTom still kept shutting itself down. I switched it back on a few times, and sometimes it'd stay on for a little while, while at others it would immediately shut down again.
By the time I neared home, the lights were back on again. TomTom had stayed on for most of the journey, but decided to switch off a couple of times, even with the car lights on. I also noticed that the battery was at around 3/4 capacity when it had been fully charged the day before. I know for a fact that I've seen it say 'charging' while on the docking kit (though you have to go into a particular screen to see it.)
It's obvious that one of the connections is hooked up to the wrong source, but I don't know whether it's the permanent live or the ignition switched one. To confuse matters more, there's at least one more wire I forgot about - the one that works the headlight dimming. I'm not particularly bothered about that feature, but it's one more wire to eliminate from my enquiries.
I've asked on the MyTomTomGo forum but haven't had a response yet. I could try TT tech support, but they're notoriously useless. I don't even expect a reply, though if they did, I expect their stock response would be that I should have had the kit professionally installed. They insist on this in the documentation, in spite of the kit being no harder to fit than a replacement stereo or a Parrot handsfree kit, both of which are DIY jobs according to the manufactuerers.
Rant over for now