The headcode cover should pop out with a bit of pressure (unless it's a tight fit) allowing them to be fitted and then popped back in. You may need to push from inside though if they're really tight to avoid snapping the cover.
I just fitted mine by removing the body and then the cab units from the body, giving full access to the back of the headcode glass panels. Quite fiddly getting the codes in place but got there in the end (after lots of naughty words when I got the first one upside down!!)
I had to laugh about using lots of naughty words, I have thought of installing a swear box in my railway room , my mate who was helping me solder a most difficult switch just so happened to grab the wrong end of the soldering Iron , I think I could have put a substantial deposit on the forthcoming Bachmann Blue Pullman, yes the air was Nanking Blue.
I have a container of water so were able to cool him off a bit.
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