nuport

Can anyone who's upgraded their sound system in their rx300 please give me some advice? I have a 2001 with navigation. I would like to leave the stock navigation system and upgrade everything else. I've been reading the archives trying to get some information. I know that since it's a integrated system it will be a more difficult install. I remember reading one post saying that since the navigation has its own line level input and that the stereo has its own line level input it can be difficult to use an aftermarket amp but I can't find that thread anymore. Can someone please help me, possibly post a diagram of the stock lexus system.
What options do I have to get around this problem?

Ideally I would like to replace all the speakers, using new components in the front. (does anyone know what size the tweeters are and can anyone recommend a component set which has tweets that can fit in the stock pods?)
I would also liek to add a sub and it's own amp.


Also are there any other snags that anyone has hit that they can warn me about?

Thanks for any help in advance!!