they are not original kit, they are a variety of after market truck (24v) ones. A lot of us have fitted them for the sake of extra safety when on the move.
as I'm sure one of the others will remind me however, the law says it is illegal to use them on a vehicle with a speed >25mph. The law is an ass, the relative speed difference bewteen Ferret and normal stuff on A roads or motorways means it is sensible to use it.
fitting is anywhere you want, there is no "correct" place as they are not an original fitting, Similarly power source is from whatever takes your fancy, if you want fancy then fit a relay for the switch, otherside just do it direct from any ignition controlled source, or if really poor just plug it into the aux socket