Since the process of obtaining the warrant would involved going through the judicial process and presenting your reasoning for why you need it in front of a judge, I see no problem with allowing it. GPS information from cars acquired under warrants could be the meaning between life and death for a kidnapped child, for example.
I believe that, in all cases, electronic or otherwise, the police and other authorities should be required to get a warrant when they want to conduct any type of search, surveillance, or monitoring of citizens. This is really the point of the fourth amendment: to protect the citizenry from constant, intrusive prying by the government.
To simply randomly attach a GPS device to a car should not be allowed, but if there is reasonable cause to believe that a person is going to commit a crime, or perhaps say a pedophile has been prohibited from being near locations where children are (schools, etc.), I think it's a great idea for a GPS device to be attached to such a person's car, just as an ankle bracelet is attached to a person.
Any kind of search must be approved by a judge or it is inadmissible in court. This includes following a car using a GPS devise. I do, however, believe in certain times, it is important to attach a GPS device, like in cases of kidnapping. However, if it is a false search, it infringes on the rights of people.