A thermoplastic composite liner for use as an inner wall of a trailer. The thermoplastic composite liner includes a glass reinforced thermoplastic sheet permanently affixed to a glass reinforced thermoplastic scuff panel along a length of the scuff panel by ultrasonic welding. The scuff panel is positioned relative to the thermoplastic sheet such that an edge portion of the thermoplastic sheet overlaps an edge portion of the scuff panel along the length of the scuff panel, the ultrasonic welding forming a weld joint between the overlapping edge portions of the thermoplastic sheet and the scuff panel along the length of the scuff panel. Preferably, the thermoplastic sheet and the scuff panel are fabricated of fiberglass reinforced plastic, such as polypropylene resin reinforced with glass fiber and the glass reinforced thermoplastic scuff panel includes fibrous material on one side of the scuff panel, the scuff panel being positioned such that the fibrous material side of the panel faces the thermoplastic sheet during the ultrasonic welding.