Boom Boom, Thanks for a well-written and informative posting, but I think that you may have missed the mark a bit with regard to vehicle searches and particularly with regard to the need for a search warrant for officers to conduct non-consensual searches of vehicles.
With just a few exceptions, an officer possessing probable cause to believe that there is evidence of a crime contained in a motor vehicle may search the vehicle as extensively as he/she could with a search warrant, but without need of obtaining a warrant. Please refer to the U.S. Supreme Court's decision in U.S. v Ross. It's also worth reading Justice Marshall's pointed dissent to that decision. A vehicle search under Ross is limited in scope to articles that fall within the probable cause.
It's also important to note that when a vehicle is impounded, the officer has legal standing to conduct an inventory search of the vehicle without need of a warrant. The extent of an inventory search is quite broad. Please refer to the U.S. Supreme Court's decision in South Dakota v Opperman.
Searches under these authorities are very commonly conducted by LE officers in the field. My experience is from California. California law permitted (when I was in active service) vehicles driven by unlicensed drivers to be impounded and the corresponding searches often produced evidence of unrelated crimes. California law includes traffic infractions within its categories of "crimes." For example, a vehicle being driven with an inoperative brake light would provide "probable cause" to inspect the brake light wiring.
This is not written from the viewpoint of police as to what they can do but rather what maximizes being able to have the fruits of the search excluded from trial. Essentially consenting to a search eliminates the need for the police to justify a search to the court. Absent consent or a warrant, the police and prosecutors are to be held responsible if they do not meet probable cause by the evidence being thrown out. Any criminal attorney worth their salt prepares a general motion to suppress evidence and then fills in the blanks with the particulars. With consent, the attorney might as well throw away the motion.
So you mentioned
exceptions (such as inventory, special places, borders) to the warrant requirement, but if you consent to a search, you have great difficulty challenging the result in court because there is no longer a warrant requirement or probable cause. The validity in admission in court of the inventory search turns on whether or not the original stop was valid (for example, see Michigan v. Long). Not a valid stop, then inventory search is excluded generally (there are some weird circumstances where it might be so). If you consent to the search, it becomes more difficult to challenge the predicate to the rest of the fruit of a full on search. Don't consent, the police have to have either a warrant or probable cause at that point in time. If no warrant, the police may request one and today makes it pretty easy to do so--even on the side of the road under many jurisdictions, but if they proceed without one, then the police officer's actions are to be scrutinized by the court in a suppression hearing as to whether the court has an independent view of whether probable cause exists. That hearing would go very differently for someone who consented to a search (then the burden would be on the individual to prove a) they could not give willing consent or b) extreme circumstances such as conduct by the police that would "shock the conscience" of the court.
As an individual, there is literally no reason for innocent individuals
to consent to a search under the 4th. A guilty person knows whether or not they are better off cutting a deal by consenting, an innocent has no such issue.
If the police
believe that they have probable cause, they can arrest you and conduct a search incident to arrest. But, after Belton v. NY, the police abused this discretion of grabable area derived from Chimel v. California that they considered the whole car as incident to arrest (it is not unless named in the warrant as some jurisdictions combine arrest and search warrants). Arizona v. Gant, reined in the police using this tactic.
If you consent and they find anything during that search, you will not be able to exclude evidence absent extraordinary circumstances. If you do not consent,
then the police must provide that their search was both reasonable and compliant with probable cause (or an exception to the warrant requirement). And no, most courts would frown on a brake light stop turning into reasonable cause to search the wiring. That is baldly a pretext search and if you use that as a general pretext to go into a car and tear it apart, that department would probably end up getting sued eventually because police have no demonstrated expertise in auto repair. I personally know of a specific case where a cop got fired for doing a profiling, stopping two yutes, and then proceeding to tear apart their car searching for drugs. None were found and all was captured on his cam. The kids complained to the supervisor, the supervisor watched the tape, the officer was suspended and then fired after an investigation. The kids got a check to pay for their damages from the illegal search.
So, if you do not consent to a search, the officer must then prove that probable cause existed at the time of the search and the reach and reasonableness will be examined by a court, not a police officer.
Impoundment inventory searches are an exception to the warrant requirement as you noted above but generally speaking, the police cannot impound a vehicle for any reason--it would have to an offense commensurate with the vehicle not being safe to drive, that was evidence in a crime committed with the vehicle, the driver would not be able to drive (being arrested etc.), and so forth. Speeding, for example, would not imply a general right to impound a car and courts have rightly rebuked jurisdictions that practice it as a regular seizure without those conditions.
Officers are given by the public a great deal of necessary discretion in how they do their jobs and even qualified immunity from lawsuits. However, their evidence gathered without a warrant is given no such leniency. We, as a society, have chosen not to follow Great Britain's laws which makes police themselves individually liable for breaking the laws during searches, arrests, and interrogations, such as trespass. They have no exclusionary rule such as we do for that reason. It is the exclusionary rule's purpose to protect citizens as Justice Cardozo say, the person goes free because the constable blundered. That is our choice as a society. Giving voluntary consent takes that away from an individual.