This thing. Driver’s door window.
When I bought my new-to-me car it went down perfectly well, because the force of gravity is not to be underestimated, but returned from whence it came with a speed akin to that of an arthritic* snail. It’s been getting worse and on Saturday needed a helping hand. New motor? No, not usually. Silicone lubricant in the frames front and rear.
Top tip: Google “silicone lubricant” with caution. Mr Google has it in mind that you need this for, er, recreational purposes.
So you buy a spray, but you don’t just spray it randomly because it’ll go everywhere, you spray it on to a piece of cardboard and then slide this up and down both frames repeatedly.
Result? The window now goes up as well as down, albeit slower.
Applies to all sliding windows, even those not on a Smart.
* Yes, I know snails don’t have joints and are unlikely to get arthritis. Never mind. They’re hermaphrodites and therefore lead interesting love lives.