Will add a little to what Don stated.
Yes the clips were hard to find but someone did repo them and they are available on ebay not cheap but less that originals were going for.
As Don stated actually the 1970 to 1972 are the same type switches.
When they get removed there is quite a bit of wiring you will need to pull. I would actually get the whole harness if I could. The relay for the PW is mounted to the LH shock tower on the back side and is usually missed.
The wire harness going to the doors it you have PW and door speakers is the hens tooth of all. The speaker wires and PW wires are in one unique harness.
The window glass in the doors is unique to PW and you have to have it. There is a big arch cut into the glass to clear the PW works. I think the rear glass is the same you just have to have all the tracks for a fastback to go to PW. Of course you can have tinted glass or no tint to also look out for.
The 73 is a one year only PW set up. The switches are lots different and the bezels have a screw with couple prongs that hold them on. The motors and rebuild kits are everywhere since same motors used in lots of Ford cars. The rear motors or pancake motors are hard to find and usually need taken apart cleaned greased and contacts worked on. West Coast Cougar has a video on their site on the rebuild.
There are lots of little clips, screws, press nuts and thing that people leave behind when removing the PW set up.
The rubber boots from the A post to the door are hard to find in great shape. If you do not have door speakers already you have to hole saw the holes in the A post and front end of the door.
The holes in the door panels have a square shape that fits the PW switch that you need to copy and the rear panels also have square holes.
I have two 1973 cars with PW and one 1972.
I lost most of my pics of the PW set up have some of my 73 Mach 1.
I caution you not to buy a partial set up thinking you will find the other pieces. NOT going to happen. I have been looking for years for a couple parts to wire harness and no luck. Always a day late on reply when they do come up.
If you can locate a parts car with the windows in you can get it all with one shot. There is one on the facebook page now but do not know how complete the PW set up is.
The pics are of 73 model.