Hi Andy. Sadly this is one of many faults with the truck. At least yours opens, mine once did, but it’s so tight now it’s grinding away at the nylon tailgate liner, and you need a finger under a corner to wriggle it open. Mine used to pop open at night randomly, setting off the car alarm. Took to MB on a number of occasions and they have no idea what to do about it. One guy on another forum got his whole canopy changed, but didn’t say if that helped later. They don’t make the UK handed canopies now, so you’ll have to make do with a LH drive model, with the opening window onto the road. That’s not a big deal as mine was sold to me like that, and to be honest I’ve not needed to go into that window parked by a busy road.
Sorry not to be more positive on the point. I have the same push sensitive lock as you. In winter, when frozen, give it a good bump all the way round on the outside, where the rubber seal sits on it. This breaks the frost and you can open the window easily, without ripping the rubber.👍