Hello, I hope the following assists you.
This will be due to the wiring becoming damaged, usually in the section that passes between the vehicle body and the tailgate, this is why when you open and close the tailgate the short circuit is broken and the light goes off.
Check the wiring, especially inside the tube that flexes as the tailgate is operated. If this section is OK then check the wiring inside the tailgate for any sign of damage causing a short circuit.
Start with these points, let me know if you have further questions regarding this and I'll reply as soon as I'm able to.
Did as you said, no improvement. Can I please add that when I said the light strip comes on when the tail gate is closed could I say it is on (a very dim light). I can make it go off fully by depressing the switch in the tail gate catch that puts the interior light on and off.It does come bright when you depress the brake pedal.
if the light is on dim and it is affected by the tailgate switch then there must be a short circuit between the lamp and the tailgate switch or a poor earth for the tailgate switch that is back feeding through the high level lamp to earth.