I have three rear glasses and I’m not sure which is for my ‘72?

Did ‘72 come with defroster in rear glass?

Mine did

Some did, southern states destined cars usually didn't have them. Only a couple bucks of parts and labour, but lets say that X 50K of production was southern states, adds up. Typical of VW, they made a couple bucks extra on southern cars.
The wife's '72 and my '74 both have the rear defroster.

Check for wiring...

Do you have a rear window defrost knob on the dash?

Check for continuity of the sliver color lines. 95% are bad.Put volt ohm meter across the spades on the window, see if it 0's out. Most will have a high open.

as an option, yes

IIRC, you could order it on a new car as an option--I think for less than $100. I retrofitted my car (early 1972) years ago while stationed in Seattle area, but I don't think the wiring was there. I salvaged wires, switch, etc, from a wrecked car.

Good idea. I assume the defroster wire comes out of the center tunnel? Here is what I have. I’ll have to dig into it.

I think all early chassis had the wiring in the harness.
Every 6 I've worked on had it at the back of the tunnel.