You generally cannot search a vehicle's trunk under search incident to arrest. However, you can impound the vehicle during the arrest, at which point it is subject to an inventory search, without violating the Fourth Amendment.
And the police have (overly) broad discretion in conducting inventory searches, which are also practically carte blanche. However, there are limits; for instance, evidence from a video tape viewed by police during one such search was thrown out.
Very interesting. I'd love to read the case on the video tape admissibility. Do you remember the name, or have any information about it?
Edit: Can't find the case, but the rationale according to a secondary source is "The search was invalid because the viewing of the tape was unnecessary to ensure its return to the defendant and it did not further any valid objective of an inventory search."
So the rationale hangs on the definition of the "valid objectives" of an inventory search. The "objectives [of an inventory search are] preserving the property of the defendant, shielding the police against claims of lost property, and protecting the police and others from any dangerous objects.
