If they are like on my bucket seats you should have two pieces: retainer mechanism and plastic cover. I cut the hole after the foam and cover were installed, and didn't trim more than enough for the head rest to slip in there. I think it looks better if you press the foam down with the retainer and plastic trim piece, as opposed to trimming the foam around them. Much easier to do too.
You could possibly cut the foam before putting the cover on, but I think it's safer to do it after covering. You will most likely have to move, stretch and adjust the cover qite a bit, so chances are that you'll move the foam as well when you do that. I wouldn't trim or cut anything until the covers look like they're where they need to be.
If I remember correctly, there is a plastic sleeve or guide attached to the frame (not really a retainer). Then you screw the retainer (an unfinished metal piece with a spring loaded catch) to the guide, then the chrome plastic cover is screwed to the retainer.
You cover the guide, but I really don't know if you need to glue the foam to it. It sounds like your'e using aftermarket foam buns, but I used foam that I bought by the yard. I made it big enough to fold over the top and the sides of the backs, then I tucked it in behind the frame and attached it with a few hog rings to keep it in place while I pulled the cover on. Just do what you need to do to keep it from moving around while covering it.
My foam was about 1/4" thick, and allowed me to screw the retainer and cover to it, without having to trim the foam. I'll see if I can find the pix I took, it might help to explain a bit better.
BTW, if you have the same type of retainer as I do (goes on the outside of the vinyl), it's a good idea to file the bottom edges a bit to keep them from cutting into the vinyl when you tighten the screws. Mine were pretty rough and had a sharp edge at the bottom, so I used a Dremel tool to break the edge.
Ok, this is what I have. Unfortunately, it seems like I don't have any "after" pix, but I hope it will do:
Picture 1: Stripped back, with the plastic guide (the black piece)
Picture 2: Top of seat with the retainer (the chromed cover is removed). You can see the guide under the cover.
Picture 3: Top of seat with the retainer removed
I'm not sure what your seats look like, but you should have something like this. At least you can see how much (or little) you have to cut. As you can see, the original padding was made of cotton, but there was also some foam attached to the back of the cover. I think there was some foam on the back of the new covers as well, but not as much. With the 1/4" foam I used, the thickness looks similar to the original around the retainer.
BTW, a digital camera is great when doing this. I had to go back and look at the pictures several times before I got everything back together
A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Come join the discussion about restorations, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!