TF small back window glass install?

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I think there's a lawyer involved in the warning. I once found a warning on a set of belts that said "Caution: Do not change belts while engine is running."

Suction cups aren't perfect but a suction cup applied to a smooth piece of flat glass has an extremely good chance of working as expected. Remember that it's all about the seal that's created between the rubber cup and the glass. As long as the seal keeps the atmosphere out, the suction cup holds. I use a little water mixed with dish detergent applied to the cup to enhance the seal. In a pinch I've used a bit of WD-40. I use them because they are water soluble and don't ruin paint. A light oil like a vegetable oil can work well also but you will lose the friction that prevents the cup from sliding around on the glass. That usually brings some "fun" surprises.