how much upgrading can I do to my factory 03 RL sytem I don't want to replace it.

thanks in advance

 

Pretty much the sky is the limit. With new processors from Alpine (vehicle hub and imprint) Rockford Fosgate and others, you run add all the aftermarket you want and still retain full factory functionability. It mostly depends on your budget and how far you want to go. Even with my 06 Infiniti FX35, which has a factory radio that you can not replace, I have thought about a full 5.1 system with the Alpine vehicle hub.

 

if you want to upgrade your system, you're going to have to replace it. you can keep the stock head unit but to see any big difference you'll need new speakers and maybe an amp to power them.