No part states it explicitly (such technology didn't exist when it was written). People who are arguing against this practice cite the 4th amendment, which reads as follows:
"The right of the people to be secure in their persons, houses, papers, and effects, 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."
People are arguing that putting a tracking device on a car constitutes a search.
The arguement is whether this is simply the same as reporting your position in public.
Or by tracking everywhere you go and correlating that with tracks of other known suspects and then extending that to tracking people that you travel near they are approaching "unreasonable search and invasion of privacy".
For example you can hardly object to the police office outside the bank seeing your car at the scene and radioing that in - but that's different to having a police officer follow you around 24x7 and asking the name and address of everyone you speak to.
