So I have to replace them . I put the very first Kenwood MP3 deck in when I first bought the car so I it should be pushing about 22 watts RMS, 50 peak.

This is in my 98 WR Blue (is that what it's called? the sky blue) 2.5RS. I'm leaning towards these, partly because they match the color of the car: http://www.crutchfield.com/S-FRpmGAL2xiJ/cgi-bin/ProdView.asp?g=400&I=130TSG1340

but it's saying I need to use some "speaker adapter bracket". Is the mounting too deep, too shallow, too wide? who knows?

Usually there isn't a problem with space for speakers in the rear. It may have to do with mounting holes. Crutchfield knows there stuff, I would ask a tech there about the situation.