there are screws on the bottom too. as well as hidden ones behind the pull handle (the one you pull to close the door). you have to CAREFULLY pop off the long horizontal plastic piece and there are screws behind that. just carefully tug on the door in different spots and you'll find everything. remember, if you feel like you will have to force it (pull hard) then something is still in there.
yes, you can use the factory wires. i was actually quite impressed how thick they are for stock speaker wires. im not sure which is the positive and negative though, if it really even matters..
supposedly, if putting in aftermarket speakers you should bypass the factory amp so it will sound better. that's located right behind glove box.
ps remember to connect the speaker wires to the speaker terminals REAL good cuz that is a spot in which they LOVE to come disconnected. nothing like having to pull the door panel off again to reconnect the speaker

try to solder them if you can, for the strongest/best connection. it's very easy to do and you can buy the materials for next to nothing.