Disconnect the speedo cable at either end.
If trans end, attach drill motor and turn on d/motor and view speedo.
If speedo works, then remove bullet in trans, device the cable screwed onto, and see if gear isn't stripped. Replac if it is, same color and tooth count.
If speedo end, drive car and see if cable turns.
if cable turns, bad speedo or not connected correctly.
If cable doesn't turn, see above.
If aftermarket radio, probbaly is since you said CD works, is battery power - red wire - connected all the time? Yellow wire is turn on power. Battery power needed for memory, clock, and to allow turn on.
Red wire is battery power all the time, no switching.
Yellow wire is switched power, ACC power from the ACC spade on the fuse panel.