The short version is that nothing available will plug into the factory amp, and adapters can be hard to come by, so you're best off replacing the whole system. The amp isn't that good anyway, and many current head units put out as much power with better sound quality than the whole stock system. If you want to keep the amp, some splicing of the DIN cable will be required. The speakers are pretty low quality too. A better head unit with stock speakers still won't sound all that good. Unless you're looking for ipod/MP3 capability, just upgrading the speakers greatly improves the stock system.