Bought this 15 year ago to keep battery charged up. I was one of the ones that complained that it would ground out the coils if the battery was fully charged. They offered me a full refund. I decided to keep it. I have it installed after the light switch and before the lights. I'm running regular 12 volt bulbs in parallel Seems to be working with no ill effects
Here is the original accessory that came on my T. Almost does the same thing I think. Never tested it.