Yes.

 

The tweeters and front door speakers are wired in parrallel (or series, can't remember) so the HU sees them as one pair, left and right. The back speakers get their own channel. So, adding an aftermarket HU, is no biggie.

 

Front and rear speakers are 6.75", keep that in mind when getting speakers, as you'll need adapters to use 6.5"

 

Cool. My old car had 5-1/4" speakers.

If I get new back speakers I might look into what I need to do to install some 6x9s.

Mike