For those of you asking if we think you can put the deck in yourself, the answer is probably. Gettting the dash apart is the hardest part. It varies from car to car. On my '89 Celebrity, it was very difficult. On my '92 Sentra, it couldn't be easier.
You will do just as good of a job yourself as some of the bonehead sleazeballs at places like BB and Audio King.
If your radio is a standard radio size (single DIN), the new radio should bolt right in to your factory mounts.
You need a wiring harness adapter for your car, which you can get at wal-mart for about $6. You will also need about 25 butt connectors (blue color) and a wire crimper from a hardware store or place like Radio Shack so you can connect the wring from the radio to the harness adapter. For those with Nissans, you will most likely need a Nissan Diversity Antenna Adapter, which they sell at sounddomain.com for $10 shipped.
Some cars have wierd setups, like a Bose system with external amplifiers and whatnot. Wiring these is somewhat more difficult, you have to bypass the factory amps or convert the output of your new deck to line level, but it's still not too difficult to figure out.
Installation instructions are usually clear, wires are color coded and diagrammed, if you can build pcs, you can install a car stereo.