Installing the vent window rubbers on the 51-5 cars can try the patience of a saint. The trick I learned to get them to seat in the grooves is to first, get them hot...I put them in hot water for a few minutes, to soften them up. Then, a thin application of dish soap will make the rubber slick enough to slide in place. Once the glass and frame is installed, keep it closed for a few days to "train" the rubber to stay put.
Hope this helps.
