The '71-'72 rubber horn inserts should slightly stretch over the plastic piece and the rubber has a lip which is supposed to catch along the lip of the plastic. In addition, the rubber slides under 3 plastic tabs along the center (2 on bottom, 1 on top, the yellow arrows), and it also has 4 small tabs which slide into grooves (green arrows). If the 3 plastic tabs are broken off and/or the rubber tabs are broken off, they the rubber will have to be taped on.
In this closeup of the bottom side, you can see that one of the plastic tabs is broken off and the other is JUST about ready to come off.
I was in the truck as a kid when my dad and I got stuck in a ditch, and as he was whipping away on the steering wheel, he caught the edge of the rubber and ripped it out, and it was never the same. From that day on, it too was held on with black electrical tape.
Anyway, you might pull yours apart to see if it is indeed the plastic piece that has broken or if it's the rubber part. Sometimes if the ridge around the edge of the rubber piece is split or broken, it won't stay on. The rubber piece I have is in good shape, but as you can see the 3 plastic tabs are broken. However, when I parted this truck out, it looked perfect and was tight. I don't think the tabs were absolutely necessary, but they'd sure help. Anyway, if you're interested, you can have this one.
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