I just bought a used T400. The doors from the cardbus slot are missing. There are supposed to be doors there aren't there?
So what do I have to replace to restore this?

There are different options depending on which option you have. Usually, they are part of the base misc. parts (43Y9633), but I don't see them listed in the manual for this model.

There should be two anchor holes that the pins of the bottom of a door slide into on the bottom.
If the two holes are missing, you'll need to replace the bottom cover.
If the holes are resident, you can get a replacement door and spring and pop it in there.

If you had just a smart card model, the "door" would be screwed into the bottom of the bottom cover and couldn't be missing
unless they forgot to replace when they took it apart. It's a solid piece that doesn't flap open