Mods, please move this to the how-to sticky if necessary.
Warning to 56k: pics are large size.
I found a way to add a aux input to the bose in my 95 easily and at a low cost. You simply have to add a few soldered wires on the bottom of the head unit PCB. You'll need a small switch, wire, solder, and rca or 1/8" jacks.
Here's how to do it.
1. Remove the bose unit from the car. Instructions for doing so are available here (the removal procedure should be the same for bose as non-bose).
http://www.installdr.com/InstallDocs...PDF/647021.pdf.
2. Remove the bottom cover from the bose unit. You will see this:
3. Solder the following wires. I determined where the L+, L-, R+, R- wires go by using a multimeter in conductivity mode and tracing the pins from the CD changer connector. The labels on the picture are from memory so please double check them with a multimeter. The pin out diagram for the CD changer connector was found here:
http://carstereohelp.net/wireharness_Nissan2.htm.
Solder connections labeled:
That's it! Solder your desired audio connector to the L and R wires, and install a switch between the AUX-ON and GND. Put the unit back in the car and mount the switches and connectors where you want. I mounted the switch next to the shifter and left the RCA wires loose inside the armrest console.
If you did everything right, it should look like this:
The input level was a little low compared to CD and Radio, so turn down the volume before switching. The head unit will power off when AUX mode is deselected. Also, the input source is audiable no matter what source is selected, so you have to turn the source off when using radio or cd.
EDIT: For 97-99 Units, the "aux-on" needs to be connected to "rx" through a switch, NOT a ground! (Thanks Nismo3112 for discovering this).
Feel free to PM me if you have any questions.