Ok this is what I did and it was really cheap. I have a dope Alpine cda7949 from my old system which has no built in amps so I needed to use the weak stock amps. I also went to a car audio shop and they gave me some ridiculous price that insulted my intelligence so I decided to tackle this on my own.
What you want to do is go to good guys or local big consumer electronic store that has somewhat of a car audio department. Go to the back where they do the install and buy an 1) adapter for a nissan antenna to the kind that your aftermarket head unit 2) The brand may vary but I bought a signal line converter for around 30 bucks by a company called Periferal ( spelling may be wrong). Its a bulky box about 5"x3"x1" has a bunch of colored paired wires and small holes on the top with recessed screws which you can adjust the gains (volume).
If your head unit has RCAs what you want to do is buy a cheap set of rcas or some you have sitting around. Cut them at the wires so you can splice them to the colored ones of the line converter. All you got to do is hook up the head unit and match one RCA to a set of colored wires on the converter when the head unit is on. Set your head unit only on the speaker your trying to match. For example: For the front right speaker, set the Balance on your head unit to the right only and set the Fade to the front. There's only four pairs of colored wires left so it doesn't take too much experimentation. The Line converters come with clear strait forward instructions. What it doesn't tell you and I highly recommend is after you get everything running and are about to bolt everything together try to wire/tape down the line convertor to something. If your car is droped and bounces that box, in not tied down will smack around and make a bunch of noise.
Putting the head unit in can be a little tricky too. My alpine trim piece fit well but I used a few washers to push the unit forward enough for the trim to piece to catch. Hopefully I'll have pic of my system soon. I love car audio a lot and would be bold enough to say that I plan to have the best quality sounding system in a P10. You'll see...hope this helps.