How would I go about hooking up a modern cd player to the wiring harness in a 72 demon with just an AM radio?

Black goes to ground.
Red goes to the power feeding the current AM radio
Yellow goes to battery on all the time (not switched) make sure it has a fuse.
Blue (amp signal) won't get used.
Speaker outputs...run to which ever speakers you add (package tray, or? ) If you are only going to keep the dash speaker, you might consider running a small pair of 3.5" speakers under there and run your speaker wires up to there.

That should get you going at least, pretty simple really

one power wire will have to goto a constant 12V source, the other power wire will have to goto a switched power
source. also, most CD players have wire that you hook up to the light switch, which will dim the display when
the lights are turned on. i would also consider wiring up ALL the other wires too. incase you wanna upgrade it further.
side note: the factory speaker wires are way to small of a gauge to take advantage of modern
CD players.
i would just run seperate wires to the speakers.. it'll sound 100x better.
whatever the case, black dosn't always mean its a ground, check first.
I assume your gonna hack the dash rather then puttin it in the glove box?

Yeah I've already hacked the dash now just about to install this thing. One last question, which wire from the cars harness do I hook up speaker too?

ones lime green, the other is brown or black, i forget which, but they'll be tied together.