I have a blip and go thingy for French Toll routes on my Merc. Pre covid we went to the Loire valley and sensibly I thought, I positioned the transponder on the windscreen on left of the rear view mirror. Wrong , as each time we came to a toll booth I had to remove it and manually hold it on the right hand side, to get it to work.
We are going to Bordeaux in October, so I have now repositioned same, on the right hand side, so we shall see if it now automatically works as it should.