The marker light fixing should be identical to that of a caravan.
There should be two fixing screws each end of the marker lamp lens which go through the lens and mounting pad and fixed directly into the side wall panel. Remove the fixing screws and then carefully remove the lens from the mounting pad which the pad should be held in place by waterproof sealing mastic. If you need to remove the mounting pad you need to carefully prize it up with a thin screwdriver blade which will lift from the sealant. Clean old mastic off with white spirit then replace the new marker light with fresh waterproof sealant.
Hi Tango55..thanks for your swift response, but the marker lights are on the top at the front of the low profile and unfortunately no screws are visible. Can't see any access from inside either.
A lot of vans use Jokon marker lights which have plastic tabs which are levered out gently to remove the lens, have a look at item no 371158525686 on Ebay and you'll be able to see the tabs.