After Market Stereo Questions 04 Gs300

[Joes GS300]

Ok I have looked a bit but I am hoping that some smarter people than myself might be able to help me out.

First I have a low mileage 2004 GS300 with the stock base stereo with a sub.

I am a bit spoiled when it comes to my tunes and I am not sure I will be happy with the base radio.

I have an Ecplise 5435 Navigation system from a few years ago that I can pop in for free (well free minus parts)

1. Does anyone know what wire harness I would need? Best Buy was trying to tell me that I needed a $150 adapter because it had a sub. Is this right?

2. What size sub is in the rear deck? Can it be replaced with an after market with a seperate amp?

3. Did I read right that the door speakers are 4"?

4. Is there anything else I should know?

Thanks a ton everyone