I began to experience the same issue as described by OP a couple of months ago in my May 2018 100D. After driving, I had a lot of persistent muscle pain in my right hamstring and realized that it was a result of the (defective) seat design rather than seat position - the infamous bar pushing up on my thigh as a result of my butt sinking so far down in the back of the seat. After reading through this thread (and others) and seeing pictures of how the seat is designed with a recessed wire frame at the back, I determined that I might be able to perform a relatively simple fix.
I purchased a sheet of 1 inch thick, heavy-duty (44 lb compression) furniture grade upholstery foam and cut it into a rectangular shape and then fitted the foam just above the wire frame and below the existing seat foam. This worked like a charm to essentially flatten the seating area (raises the butt area to be level with the bar and plastic front of the seat) so that I do not sink in when sitting down. No more pain.
I measured and experimented and found that a 1 inch thick, 8.5 inch deep, and 12 inch wide foam rectangle fits perfectly on top of the wire frame. Note that upholstery foam must be cut with a serrated blade, a common utility knife won't cut it (pun intended). You can buy upholstery foam from many places online - I just purchased off of Amazon.
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I did not have to remove the seats for the fix; I worked from underneath the seat:
- Move seat all the way forward so that you have room to work in the back.
- There is a flap which comes from under the plastic seat back which covers the back of the seat bottom. It is secured by two plastic hooks connected to the wire frame by elastic bands. Unhook them and lift the flap.
- Now you will see a felt flap which covers the back of the wire frame and seat foam. This is again just hooked to the frame by three plastic hooks. Unhook them and lift that flap. There are a few wire harnesses between the two flaps, but there should be enough slack in them for you to work. Just do not pull / stress them too much while working.
- Now you will see that the back of the wire frame sits on the back seat frame bar via four fairly thick plastic hooks. It takes a little doing, but you can push the hooks up and this will expose the area between the frame and the bottom of the existing upholstery.
- Take the foam rectangle and insert so that it sits as far forward on the frame as possible to create that level shelf. You will have to compress the foam and squeeze it between a couple of the wire harnesses. Once it's past the wires, it's very easy to manipulate into position.
- Reverse the steps and viola! One note is that when you unhook the wire frame from the seat frame, the front of the wire frame may move back and fall out from where it's hooked in front. No big deal. Just position the foam and hook in both front and back. You won't be able to see where the frame hooks into the plastic part of the seat frame in front, but it's easy to get a hand back there and reset the frame into the depressions in the plastic. You'll feel it hook into place.
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This all may sound like a lot of trouble, but I completed these steps for both driver and passenger seat in 1.5 hours - and I didn't even know what I was doing at the time. I could probably do it in half the time with the knowledge above. The tricky part is that you do need a flashlight and to be able to lay down in the back seat foot well to be able to work. If you are a bigger guy/gal, it may be more difficult to get a view and your hands under the seats.
This fix helped me out a lot and thought I'd share given that I've seen quite a few posts on this issue. I liked the idea of using upholstery foam rather than utilizing plastic or wood supports since it seems more natural to just add more padding to the seating area rather than adding something non-compressible. Thanks!
