Heres what your system could possibly look like....notice the connections on the amps...It can be done very cleanly...

Just a little FYI.... Im still useing my stock radio.... yet it sounds like an aftermarket! Its all on how you set it up...

 

wow that's a lot of wires haha

Why didn't you just do the line out converter? Or did you do that? Just looks like you have some extra wires.

The only large benefit of an aftermarket headunit is it's easier to change all the variables of the sound and in most cases they look better. oh and they're a lot easier to hook up

 

Aftermarket headunits do help alot when changing the variables but couldn't a equalizer be used with a stock radio or is an aftermarket needed?

 

Most stock radios suck as far as sound quality and output, but ALWAYS save it, just incase. It honestly depends on the amount of money your trying to spend. You could build a nice system with a simple 200 bucks really, maybe even cheaper.

RCA output at best buy 15 bucks
Aftermarket amp - depending on subs, roughly 50-100
two 2s or big boy 12s, roughly 50-150 or 150-250.
Or just buy a 4-6 channel amp, run speaker wires to your speakers and wala, however the amp watts need to be LOW for factory speakers..or just buy a whole new set and jam away .