US Constitution, Fourth Amendment.
Keep reading. Let us know when you get to #16.
So, let's see what it says.
"The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration."
So they can levy an income tax. And they don't have to give any of it to the states. And they don't have to worry about any head counts. But there's nothing in there about exemptions from 4th amendment protections.
So let's take a look at that 4th amendment:
"The right of the people to be secure in their persons, houses, papers, and effects,[a] against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
So if you wanna demand access to someone's person, house, papers or effects, you gotta show probable cause which is supported by Oath or affirmation and describing a particular place to be searched and the particular person or thing to be seized. And no, shoving "income tax enforcement" in front of the request doesn't seem to be a reasonable substitute.
But you are right in that the government has repeatedly exempted itself from having to follow the constitution - on this particular issue and many others. And by that I mean in all likelihood they'll be turning over the info the government wants.