Constant 12v always has ~12 volts... probe til you find one thats always reading voltage.. tap this with yellow wire from deck
switched has no voltage til the key is turned to on/acc or the car is on, so probe the wire then turn the key over to on. if you have around 12-14 volts when the key is on ...thats switched...tap this with red wire from deck
use the black wire on your cd player harness to ground it, either extend it to bare metal or if the one supplied reaches thats great.
Then just test around for speaker lines with a pop tester... and connect accordingly