I had a question.

with my older sc300 i went to my local stereo shop who's known for some great work.
and just out of curiosity i asked him how much it would cost to put in my old system from my
rx7 into my car (sub in box and amp). He said i'd have to redo everythign as lexus systems are all individually amped.

sorry for being so vague, but i was wondering what would be the best way to come about this?
can i just hook up my after market stuff to the headunit and disconnect my stock rear sub?

thanks in advance.