With the price of aftermarket in-dash GPS receivers falling below $1,000, how much longer can automakers keep charging $1,500 to $2,000 for standalone GPS navigation system? (Aftermarket products are the things you add after you've bought the car.) Check the electronics store circulars or an online site such as Crutchfield and you'll see systems for as little as $500. More are selling for $800 to $1,000. Except for the smaller buttons that are hard to press, they're about as good as what you get with a new car.

For the money, you get not just a navigation system, but a complete radio with audio/MP3/WMA disc player and on some systems a DVD video player (when the car is parked or you hot-wire the lockout), Bluetooth module, iPod adapter, memory card socket (for music), and/or the ability to connect an iPod and satellite radio.

At 6th Avenue Electronics, for instance, a Panasonic Strada CNNVD905U with 7-inch screen and 30GB hard disk drive runs $800 while a Pioneer AVICD3X with a 6-inch screen runs $1,000 installed. At Crutchfield, five systems sell for $500 to $1,000. The Pioneer AVICD3X unit with Bluetooth and iPod modules runs $800 direct. Many of these units can be had with steering wheel adapters, $100 add-on modules, that retain the functionality of your steering wheel audio buttons.

Here's the catch: These are double- or single-DIN systems, meaning, in plain English, you need a radio cutout on your dash that's 7 inches wide and 4 or 2 inches high. You'll find the DIN cutouts in some minivans and pickups, plus a handful of sedans (more often the cheaper ones). The majority of cars today have custom radios that aren't standard DIN sizes and the head units (the electronics module in the dash) may also control other functions, so if you could swap out the module, you might lose your air conditioning functionality. Also, these units may lack every bell and whistle, for instance real-time traffic information - but that's not a crushing blow, because RTTI is still in its infancy, and while it can tell you about traffic jams (usually), RTTI is not so good telling you with any degree of confidence if there's a better, faster route.

It used to be that from-the-factory navigation was better because the car's speedometer/odometer and compass helped track your location if you lost the GPS satellite signals, for instance in a tunnel, but most aftermarket units have built-in gyroscopes that perform the same functionality. If you're thinking of adding GPS to your existing car, if you also want to add Bluetooth for handsfree calling and an iPod adapter for your music, double-DIN (preferably) or single-DIN units are a great choice. If they fit into your car.