If you wanted to install a Nissan radio, you would have to install the one that came with it originally. The main harness for the Navigation System equipped Maxima would have been different from the non-Navi Bose system. The other concern is that the Bose systems, which came in the GLE's, usually use 1-ohm impedance speakers. If this was the case in 2003, then installation of an aftermarket head unit would usually require some modification or replacement to the original amps and speakers, so it may not just be a wiring issue. You best bet, if you don't know what came in it originally, would be to take your VIN to a Nissan dealer and find out what was originally installed. Once you know what it had originally, your best bet would be to try and find what you need in a salvage yard. Try Car-Part.com--Used Auto Parts Market
. It would be a good idea to pull a door panel and see if you have the Bose speakers or an aftermarket replacement and check the amps to see if they are intact and bypassed or modified or have been replaced with aftermarkets, as well. It would also be a good idea to pull the head unit and see if they used an adapter harness or hard-wired it. If they hard wired it and disposed of the harness connectors, you'll need to source them from a salvage yard, as well. 20003 was, IMO, one of the best years of the Maxima; it's a shame somebody fudged it up!