If you're still cruising around in a car with an old stereo, that doesn't mean you're stuck with old technology. Blogger and DIYer Kevin Roof dreamt of the ability to wirelessly stream his music library to his deck, so he hacked in stereo Bluetooth support.
By fooling the tape deck into thinking it had a cassette loaded, he was able to hack in Bluetooth support using a bunch of spare parts. You'll need a USB connector, an 1/8-inch jack, a power adaptor, an isolation transformer, an A2DP Bluetooth Module and a soldering iron to follow Roof's guide, but he was able to get it working on the cheap. With the exception of a switch to flip between the Bluetooth source and the radio, you won't see any aesthetic changes. If you're interested in converting your old stereo without completely upgrading your stereo, check out Roofus's full tutorial on his website.
If Roof's method seems a little heavy for your tastes, you can still integrate Bluetooth into basically any car stereo with a universal Bluetooth kit. See this primer on car-stereo site Crutchfield for more details.