I ran power and ground wires to the battery and frame then cut the output from the head unit to feed the amps input and ran the amp output into the other side of the cut output wires. There's space behind the head unit to place the amp. it's relatively small.
I've got an SR5 with base system and upgraded it with new speakers and the kicker 47Key200.4 amp. and a powered sub. I'm very happy with the sound however the sub really was overkill.
Kicker makes a 4 channel amp that's more powerful, cheaper and comes with an automatic dsp for fine tuning the sound. It doesn't come with a plug'n'play harness, so you have to do some wiring yourself. If you have any experience doing stereo wiring its pretty easy
Kicker makes a small amp and dsp that'll fit behind the head unit. It comes with a mic for automatic tuning. model #47key200.4. I believe its less expensive than the beat sonic too.