make sure the "backup" fuse in the underhood fuse box is not blown ...

You don't have the 12v constant wired up.

I know on radios it's a green wire. Not sure about the clock.
I'm assuming it has 3-4 wires.
1) Power,
2) return maybe,
3) ground
4) constant. But it could certainly have two.

Bottom line, you don't have constant power to the clock.