The holes are not behind the black trim. They are next to it and covered by rubber grommets which will have been over sprayed with paint. Look closely and you will see them. I'm about to remove mine for attaching bars.
It gob smacks me why any after market side bar would ever have a different type of fixing. My current ones attach to different points on the van, without the need to drill holes, but they are then in the way of the jacking points....
Stupid.
Surely it's far easier to just use the originals rather than work out and where else to fix them.