run some LED tape lights down the entire length of the bed under the top rail. Check if the liner stays up against the outside of the bed at the top.
You can buy 12V weather proof LED tape lights at SuperBrightLEDs.com. Be sure to clean under the rail with rubbing alcohol, and wire them to the existing bed lights (will need to know how to solder if you buy a 16' roll). Did this type of lighting on my buddies 2500 w/8' bed.
I plan to 3d print some pucks to put an angle on them so more light shows into the bed from thingyverse site. Colder right now so hesitant to use 3m tape in fear it may not last long. Might counter sink magnets into the pucks and JB weld the pucks in to bed of truck so I could replace lights later if they die out more easily since back of light housing is metal. Not sure yet, but lots of lighting options out there on amazon.