This is a recent phenomenon. I have a standard 3 wire Ford alternator fitted with one of these devices for a single wire conversion
http://store.alternatorparts.com/partnof7078.aspx
Just this morning I noticed that the battery light was not coming on when I turn the key. The battery seems to be charging although I have no voltage from the convertor to the warning light.
Anyone know of a way I can tell if its the alternator or the convertor thats faulty?