Super 2001 4 Star - 4 Horse Trailer with Living Quarters in Magnolia, AR

Many older cars took the feeds for the trailer's lights directly from the towing vehicles rear light circuits. As bulb-check systems were introduced in the 1990s "by-pass relays" were introduced. These took a small signal from the rear lights to switch a relay which in turn powered the trailer's lights with its own power feed. Many towing electrical installations, including vehicle-specific kits incorporate some form of bypass relays.