Air Compressor - Blow out the dust and dirt.
Dry Rag - Pick up as much dust as possible if you don't have an air compressor.
Nifti and warm water solution. Wash the cab starting from sides, rear and front. Use an old paintbrush to remove the stubborn filth. Rinse with a clean wet rag.
Let it dry.
Sand and repaint

Some filth just cannot be removed. Stains that have permanently stained the bottom panel need to be sanded and painted over. Clearing or lacquer will just ensure it is more pronounced.