It merely makes it the opinion of the court.

Not so. When a court holds something, it is exercising its lawgiving function. The opinion (sometimes referred to as 'dicta') is an explanation for the holding. The latter is basically a statement of views, but the holding itself has legal force. It's true that they are often mixed up, by the public, press, lawyers or even other courts in descending order of frequency; that's why there are higher courts, and even the Supreme Court sometimes reverses a previously held judgment.

The law has not changed, and the law is quite clear in requiring a warrant.

No, it has not changed, but nor is it as clear as you think. We do not know whether or not a warrant was obtained in this case, but current law (as held recently) is that if the car is accessible to the general public, then sticking a tracking device to it does not require a warrant, any more than a stakeout does.

It is very important to recognize that the courts do not create law, and they cannot change the meaning of the law with rulings (otherwise there would be no purpose in having separation of powers.)

This is so, but the courts do have the sole power to decide how the law should be interpreted, should the meaning of the law be unclear. The Constitution (III.2) says that '[t]he judicial Power shall extend to all Cases, in Law and Equity [..and..] the supreme Court shall have appellate Jurisdiction, both as to Law and Fact...'

Jurisdiction means, literally, stating the law. The separation of powers is embodied in this decision-making power of the judicial branch - Congress makes laws, the Executive makes decrees, the Courts make judgments. And that includes judgments about the meaning of the law, which is why the Constitution draws a distinction between 'Law and Fact.' If the courts were only able to evaluate matters of fact, then every dispute over the meaning of law would have to be resolved in Congress, a task which would be hopelessly burdensome.

This is sort of how it works in countries with a civil law system; courts are mainly triers of fact and apply the law without analyzing it in any great detail. But even here they sometimes make adjustments, if errors or contradictions exist which would lead to an absurd result. The United States has a common law framework, which is partly why we still have the notion of judicial 'circuits' (which in the distant past involved judges traveling around and setting up temporary courts called Assizes to judge cases wherever they stopped). Common law is where we get our notions of precedent and the reason we look back to earlier decisions (and the lengthy explanations given for them).

Why can't you find anything about the common law in the constitution? Because it was left to Congress to specify regulations for the Courts. Which they did in the Judiciary Act of 1789, during the first Congress, under President George Washington.

http://www.constitution.org/uslaw/judiciary_1789.htm

Legislation is always incomplete because it is impossible to anticipate all future situations. By resolving ambiguities and contradiction via an orderly process, Courts can fill in the gap where the legislation does not provide a clear answer. If Congress disapproves of a court's interpretation, then it can and does issue new laws to update or clarify its intent. If it chooses not to, then the Court's holding remains in force; if the public is unhappy with the result, it must elect a Congress which more closely aligns with its opinion.

For instance, the existence of the FBI is unconstitutional

Rubbish! Who told you that? By that logic the Air Force would need to be disbanded immediately, since the Constitution has nothing to say about flying machines or new branches of the military.

Go read Article II, which mentions 'executive Departments' without spelling out what they should be, and 'Officers...whose Appointment is not herein provided for, and which may be established by Law...' - are the departments or offices in question enumerated? No, it's assumed that Congress will create them as it sees fit. And if you think something they do or create is unconstitutional, then you seek the decision of a court, because only courts have the authority to make such determinations. Congress can't certify its own output as constitutional, nor can the executive state its decrees or decisions to be so.

Seriously, if this is your own interpretation then you need to do some more study, or if you got the idea from someone else then you were badly misinformed. the US constitution is the first word of American law, not the last. The idea that if you can't find something in there then it doesn't exist is terribly misguided.