Hello David,
You will need to find the appropriate fuse and check whether it’s intact or not.
If it is, you will need to check you have 12v coming to the offending bulb holders.
If you don’t, I would be checking for fractured wires in the flexible harness section twixt the body and tailgate - a favourite place for wires to fracture due to opening and closing.
Kind regards,
Gareth.