I think you're referring to the Multi Use Parts side of the Body Search die (also on Bag'a Booms).
You actually just take that die and put it in your locked slots, you don't substitute it for the Scavenger dice. When you want to make a bomb, you can tick down that die as any of the three needed parts (Element, Casing, or Fuse). You can even tick it down twice to use it as two wilds for the same bomb.
Hope that makes sense!
Thanks so much, not sure why I didn't pick up on that myself. Again, thanks for the clarification.