How do you find out what wires go where when putting a new radio in a 1993 Nissan Altima if none of them are labeled and it is not in the Chilton's manual?

Buy a wiring harness from a local installation center ( best buy etc.). Car manufacturers use any wire laying around to complete the job. The harness is only like 10 bucks. All you need to is to wire the new stereo to the harness (match up the colors) and then clip the harness into your car. --------

First, get a multimeter or basic volt meter. Generally there should be 11 or 12 wires for the stereo: (1) Constant 12 Volt, (1) Accessory 12 Volt, (1) Sensor wire, (8) Speaker wires. With ignition off, use multimeter and find constant 12 volt wire - this is always on even with ignition off - used to save radio stations and settins. Mark this wire as constant 12 volt. Turn ignition on and find Accessory wire - only comes on with ignition on or in accessory position. Put power wires into new stereo, and then narrow down the speaker wires by connecting two at a time - there are two wires for each speaker, generally the colors match up for each pair. The sensor wire connects to the stereo - the stereo puts out a 12 volt signal to either put up a power antenna or can be used to turn on an amplifier. There may be a ground in there as well.