The difference between 16 and 18 gauge speaker wires in a car stereo stystem is insignificant. You won't notice any difference whatsoever. For any practical car audio application, the stock speaker wires will always be just fine. One could come up with a theoretical situation, like "If I hooked up an amplifier that could drive a signal with 2000 volt maximum amplitude into a 2-ohm speaker load, I'd be sinking 1000 amperes through the wires, and the wires would incinerate themselves" -but that'* never really going to happen, so this is just not anything to worry about.
Of course, replacing the stock wires with larger gauge wires will also not hurt anything. So if you have time and wire and it makes you happy, then go for it.
97 SE, teflon wiper blades, Dunlop Sport A-2 tires, Leather steering wheel cover, Pioneer DEH-P480MP mp3 CDRW head unit, Pioneer 12-disc changer, iPod hookup, Boston Acoustic 3 way 6x9'*, Boston acoustic separates in door pods, Alpine 4 channel amp in trunk. Magnaflow dual outlet muffler and Ractive stainless tips. Autozone rice pipe and K&N 4750 intake kit. Clear corner lights. Energy Suspension end links.