I've been where you are right now. Buy a 4 channel amp, you dont need anything special, just make sure it has line input, not just rca input. This is what I did, and you may not want to, but I cut the stock wires going from the stereo to the speakers right at the dash. I crimped male spades on the stereo side of the wires, and female spades on the speaker side of the wires, so I could hook them back up to stock if needed. Then I ran wires from the stereo→to the amp, from the amp→to the speaker wires. This way I had 4 amplified speakers while using the stock REF radio. I even ran the pre-amp output from my 4-channel to my monoblock to power my subs. Even after I got a new head unit, I still use the stock wires for the speakers. Until I get some 6.25" RockFos midrange woofers, I am keeping the stock wires since they handle the LOUD volume just fine. As for the tweets, grab some 1" tweeters, and install them in the cardboard covers in the dash tweeter mounts (you will see what I am talking about), then tap into the front speakers to power them. It is a day/night difference, even with aftermarket speakers. To access the tweeter mounts on 08/07 models, take the 1 screw out of the A-pillar, and pull up/out to remove, then grab the side of the tweeter vent, and un-snap it all the way across the dash. Most of the interior in these vehicles are snapped together.