dont be intimidated by all the wires; the easiest part is the wiring.
u may have to cut that black plastic piece (labeled ISOPORT) to access those wires. the other wires, labeled 9-21, you will only need to use if u intend on using those specific applications (back up camera, gps, RCA inputs, etc.) -not totally necessary for basic install.
Metra Wiring Harness for Most 1987 and Later Toyota Scion Vehicles 70-1761 - Best Buy
crutchfield is an EXCELLENT resource for researching products...as was mentioned.
^^^thats wut u need for wiring any head unit to ur car. connect the wires from that to the matching (metra provides a color coded key) wires from ur new radio. then jus plug in the white harness to the factory plug in the dash...u can use crimp caps, electrical tape, twist connectors, wutever u want to connect the wires...simple stuff. bestbuy.com, sonicelectronix.com, everyone sells these
scosche makes dash kits for aftermarket radios- as was already mentioned. u can find those pretty much anywhere (online, bestbuy, sonic...etc.)
DONT FORGET TO DISCONNECT THE POSITIVE BATTERY TERMINAL...jus as a precaution.
u have to carefully pull the black bezel toward you (can use a screwdriver and gently pry the bezel off evenly) and then remove the slim lower plastic panel right below the radio...remove the bolts from the radio brackets, remove the radio, unplug harnesses, use the old brackets if you dont wanna use the shitty plastic ones that come with a dash kit (although i did and i gotta admit they work fine, jus be gentle) and fasten those to the new radio (small screws). hopefully u already wired the new radio harness to the metra harness adaptor and now its just plug and play time. scion harnesses conveniently include a ground wire so u dont have to find a spot to ground the radio.
before u put the bezel and panel back after u made ur connections, reconnect the battery and test the radio to make sure it works.