I replaced both center diffuser vents and one of the outboard ones on my wife's 190.
Yeah, they're cheesy but easy to replace. You can use a butter knife or any other thin, flat but not sharp blade. Slide it between one end of the diffuser and the dash, where it rotates, then gently press the knife against the diffuser to pry the diffuser out from its pivot hole and away from the dash. Then do the other end of the diffuser the same way.
The new part simply slides in and latches itself into the two pivot holes in the dash. Shouldn't take more than 5 minutes, tops. You might ask the dealer if it's cheaper to buy them in bulk, seeing as they only last about 10 months before they start pointing at the floor again.
1989 190E 2.6