Use 12 gauge speaker wire.
and I'm pretty sure she doesn't want to rewire. I would, but that's me.
Well, if you are going to power the speakers with an amplifier, you might as well. In my truck's case, it was easier to run all new wire from the amp straight to the door panels than it would have been to run them into the dash and splice into the factory wiring.
However, using 12 gauge wiring for the speakers is absurd. 12 gauge wire is capable of carrying 20 amps of current, that's huge overkill for any ordinary speakers. It does NOT make it "sound better," and if there actually would be an improvement between a 12 gauge speaker wire and a more reasonably sized wire, you'd never be able to notice it inside a vehicle. As long as you use good quality wire, 16 gauge wire will be more than enough in most cases. Lots of good info @
http://www.bcae1.com/wire.htm
Something you can consider is deleting the rear speakers all together and running a nice set of components up front. Our truck cabs are easily filled with just front speakers, and since you're not going for competition or anything I think front speakers would suit you fine. I am running a set of components up front with deleted rears and it goes louder than I can stand to listen to. Stays clear the entire time, too.
+1 The best rear speakers IMO, are no speakers. Your system is only as good as it's weakest link, and in most cases that would be the rear speakers. Keeping the rear speakers would just be wasting money to make the entire system sound worse, IMO.
As for the head unit I'd stick with the usuals: Pioneer, Alpine, or Kenwood.
Don't forget Eclipse and Clarion.
I'd say get a new head unit, 2 nice components for the front, a 10" sub, and a 4-ch amp to run both the components and sub. I think that would suit you just fine based on your criteria.
That is the same setup I'm using and I'm very happy with it. That would also likely be the cheapest route as well. However, I do wish that I had ran larger wire to the amp so that in the future I could add an amp dedicated for the sub(s) and use my existing 4-channel amp to power my midbass woofers and tweeters seperately.
Go to a local car audio store and take a listen to different things. Figure out what your ears like. It's different for everyone.
Best. Advice. Ever.