Q
I think you'll probably find your running on is when the fan is working and they all seem to do that.
Ben
Interesting. Seems like you are on the right lines. Could it be that the 3.5 volts is not only causing the running on, but also causing the light to remain out. Probably won't be able to get round to it for a couple of weeks, but I think I'll fit a diode and take it from there. Can't do any harm.
I wonder. Could it be that the alternator normally has a diode in it (or similar) and this has failed 🤔