Mount to the frame and make sure you allow enough wire for a Service Loop between the backer plate and the frame mounting. If your trailer has springs make sure the wires will clear all movement while on the road and allow for the trailer to be jacked up. If you have questions i would ty wrap the wires to the springs and make the service loop at the end of the spring where there will be the last movement.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles