There is no way now for them to know if they came from that car
No, not necessarily.
Evidence needs to be introduced in an admissible form as dictated by the particular jurisdiction's procedural rules - usually by witness statements establishing where the item came from (its "provenance") and a chronological account stating who had control of the item - and why if it's relevant - in what is often called the "chain of custody" or the "continuity of evidence". Any and all of these witnesses can be challenged by the defence if there is any doubt that the airbags, for example, are not the ones from the accident.
Would that be tampering with evidence?
Again, not necessarily. The airbags may need to be removed for any number of legitimate reasons: forensic examination; physical inspection; to show if they were, or were not, deployed... Again, it's up to the defence to question the reasons behind their removal.