Remove the vent grill and dash pad [panel]. Instructions can be found here .
The following instructions assume you already have the dash panel removed.
The photocell should now be within your reach. You should be able to see what is shown in the photo below. You will need a 7mm socket to remove the bolt holding the bracket the photocell is mounted on. The 7mm bolt you're looking for is in the gap between the solar temp sensor and the photocell. (Below the level you can see in the photo.)
You may not need to remove the bolt if you can turn the orange piece on the bottom of the photocell socket clockwise to let the locking tab come out the bottom. That will allow the round black part that houses the photocell will come out the top.If you don't have enough room to get to it, then remove the 7mm bolt and the entire plate that holds the temp sensor and the photocell will pull up a bit as far as the connecting wires will allow. Then you can get at the photocell and remove it.
The photocell plugs straight into the black socket. It does NOT turn like a marker light.
Don't forget the blue filter if your replacement doesn't come with one.
Some people have had good luck getting a photocell from Home Depot. I would recommend either GM or a used one from Jim Finn. Click here for more information about the Home Depot photocell.