A man’s home is his castle. The car, maybe less so. Still, the Supreme Court slam-dunked overzealous law enforcement with a 9-0 decision on Monday that said cops need a warrant before attaching a GPS homing device to a suspect’s car. They remain free to track a car the old-fashioned way, by tailing the other guy and not being noticed, or by typing up a warrant request and finding a friendly judge. Before civil libertarians and drug runners breathe a sigh of relief, bear in mind: This is just one case where technology that is digitized, downsized, wireless and ubiquitous redefines privacy when you’re in your car or otherwise away from home. More cases are coming. Some that were decided could flip with different judges on the high court.

How else is your privacy in flux inside a car? Did you know a telematics system such as OnStar can listen in on your in-car conversations? Yep. Federal and state authorities know that and they’ve tried to take advantage, with mixed success. For this, there’s no question a warrant is needed: It’s a wiretap, plain and simple. Nearly a decade ago, one judge rejected a wiretap request to record the suspect’s in-car conversations but only because the wiretap grabbed control of the integrated cellular module, which disabled emergency crash notification.

Black boxes, Ford Sync, and E-ZPass toll tags

Every car has a black box data recorder, like commercial airliners have, that constantly records vehicle status. It’s helpful when the dealer has to diagnose an intermittent engine fault. It can also record car speed, time of day, wiper status (wipers on, suggesting poor driving conditions), outside temperature (suggesting the rain is turning to ice), stability control and anti-lock brakes invoked (possible out-of-control driving condition), and seat belt status. In an accident, the vehicle data recorder could show you were a model driver, or it could show you were doing 15 mph over the limit, weren’t wearing your seat belt, and didn’t hit the brakes until a quarter-second before you hit the other car. Most automakers’ terms and conditions say they won’t reveal the contents of the black box with a few glaring exceptions: If the police want it or if you sue the automaker.

A variant of the black box recorder lets you get cheaper auto insurance if you agree to limit when, where, how much and how fast you drive. For people who must have a car and can’t afford high insurance premiums, it’s a mixed blessing. Other motorists worry that insurers are biding their time until they can make metered insurance the norm. On the other hand, there are limits: A small car-rental firm got slapped down by a Connecticut judge when it used the black box to fine (surcharge) a renter for driving over the speed limit. The judge said Connecticut gets to make and enforce speed limits, not private companies.

The same privacy incursions you don’t want applied to you, you might like applied to your teenage children. Telematics services (OnStar, BMW Assist, Audi Connect, Lexus Enform, Mercedes-Benz TeleAid, Hyundai BlueLink, and Ford Sync) have or soon will have geofencing that sends alerts if the driver strays behind a designated driving area, or goes too fast. Originally they were simplistic (speed limits were absolute, not relative to the posted speed; geofences couldn’t easily be expanded on weekends) and some services surcharged for every message sent. Not every teenager likes this, but almost all agree it’s better than no car at all.

The real-time tracking by OnStar and competitors allows for stolen vehicle tracking, which is good for everyone but the carjacker. But early on in OnStar’s life stolen vehicle locater services got used for errant husband tracking, which didn’t make for good customer relations. (Note to editor: Good place to insert photo of Newt Gingrich, Mark Sanford, Anthony Weiner, or Jon Edwards, or…) Now if you want to find out where your vehicle is via OnStar, you’ve got to file a stolen vehicle report. But as telematics services push real time tracking in the future and supersedes one-time location reports, the cheating spouse may want to choose a non-telematics car. Or walk. Just hope the feds aren’t tracking visits to Ashley Madison.

Many states offer toll tag readers that speed daily commutes as a tradeoff for the government having a record of where your vehicle is. It’s voluntary, if you don’t mind longer delays at the cash lanes. So far the states have deferred from calculating your speed between tolls for the purposes of issuing speed tickets and enhancing revenues. (Not so in parts of Europe.)

Next page: Automatic license plate scanners, vehicle-to-vehicle comms, and the future of in-car privacy