Chevy Venture only CD

Replace the factory head unit with a CD/Cassette combo deck such as the Sanyo FXCD-500.  It's not the highest quality deck in the world, but it would give you what you want and it's single DIN sized.  There are several options in the 1.5 and dual DIN categories.

Oops, sorry about the post above.  It went in by mistake and now I can't delete it.  Oh well!

Yea, I played around with one but was not too impressed.  Sanyo does make a much more expensive version called the FXCD-1000, but it probably uses the same transport mechanism.  The only other options are equally unattractive in the 1.5 DIN size systems.  There are CD/Cassette combos available from marginal companies like Pyle and Jensen that MIGHT be more reliable than the Sanyo.  If you can squeeze a dual-DIN into the Chevy, then things get more interesting, with offerings from Kenwood and Panasonic, and if you want to go all the way, McIntosh makes a dual-DIN CD/Cassette head unit. 

Of course, the BEST thing to do would be install a CD/MP3 player and sell the customer a CD burner and tell them to dump their cassettes onto a disk... but I guess that's not exactly being customer-friendly, eh?  :)

Good luck.