The best bet is to install another complete unit, and leave alone the factory components. They make universal DIN boxes that are sold at most stores which carry car audio, and definitely at trucker supply and RV delaers. Or, as others said, you can mount it someplace out-of-sight, as you might want to with a classic car that has the original AM radio for show. Just get one with a full-function remote. Or if it's a later model car, somebody probably makes an adaptor plate for it. The GM full-size trucks & Suburbans were a big problem for aftermarket stereos in the late 80s to mid-90s, since they were "control head" style with the radio box down deep, and only the controls & faceplate up on the top face of the dash. Most newer car & truck stereos, even if they have a weird-shaped face, have a DIN or double DIN chassis right behind. In those cases, you can usually get an adaptor face from eBay or a stereo shop. Lexus has a very weird setup, but in that case, it's a great factory stereo and no reason to upgrade, IMHO. Same is true of our Silverado 4x4 Diesel. It has a great factory system in it with backseat DVD player and all.
Charles