2007 camry, 4 channel amp?

Hey guys, you have proved invaluable in the knowledge department when i installed my manual trans. remote start last year. I have a 2007 Camry CE that i will be installing an older Sony 444W 4-channel amp and 2 pairs of Alpine Reference 6030CS component speakers w/ tweeters with crossovers. I have already upgraded the head unit to an Alpine CDA9886 and also am running a 300W Cerwin Vega mono amp to my JL 12w3V3 sub in the trunk. I plan on starting with fresh wires. 4G to a distribution block, then 8G from there to the amps. My question to you guys is this. To wire up the components, i believe i will run wire from the 4 channel amp to the crossovers and from there to the speakers. Where is the ideal place for me to mount the new amp? The mono amp is currently installed on the back of the passenger fold down rear seat. Next, will i just be running new wires to the speakers, or will i be tapping into the factory wires? If tapping into the factory wires, where is the best place to tap in? Also, where is the ideal location for the crossovers. I am no dummy and take instruction well, i just have never wired speakers to an amp and components are new to me alltogather. Thanks for any advice and help here guys.

In case anyone who looks, cares, the job is complete. Crossovers installed in front doors. Rear crossovers installed on rear seats with the 4 channel amp. New 4 Ga wiring from the battery and a distribution block installed. All fresh speaker wire ran to all tweeters, speakers and crossovers. Power down the passenger side and 3 new sets of JL Audio RCA's down the middle of car. Looks good and sounds much better. You don't realise how much power the hu uses to power the speakers. INCREDIBLE!!!