1) The police don't give me my Miranda warning and get me to confess and I say I left the murder weapon in the trunk of my car. They get a warrant. They CAN use the gun against me. Yep, that's right. There is NO fruit of the poisonous tree doctrine for 5th Amendment violations.
2) The police give me my Miranda warning and I shut the **** up. They act on their hunch and open up my trunk. For the purposes of this, this is clearly a 4th Amendment violation (no exigent circumstances, no search incident to arrest covers it, no inventory search covers it, what have you). They CANNOT use the gun against me. There IS a fruit of the poisonous tree doctrine for 4th Amendment violations.
It goes even farther. Let's say this prosecution gets contentious and this jackass takes the stand. He confessed to the bombing and knowing he had a bomb but on the stand says no, I didn't do it -- that all he did was carry the backpack and drop it when his brother yelled at him to drop the thing. He claims he had no idea there was any bomb. His statement obtained without Miranda then becomes fair game and can be used to impeach him. The Miranda right is a very weak right in practice. That said, if they can't prove (I think it's probable cause to think there was) an ongoing emergency and if he does not testify, they won't be using any statement he makes in interrogation against him.