Note: if you are really interested in this, you may want to bookmark it and come back in a week or 2 as I finish the install and flesh it out more.
I have a 2001 Nissan Pathfinder that I finally decided I wanted built in iPod control and Bluetooth. My brother recommended Kenwood, and I really liked his from about 6 years ago. After seeing the Kenwood KDC-742U was getting pretty bad reviews on Amazon, I decided to go with Pioneer. The DEH-P7000BT got great reviews, but the brand new DEH-P7100BT had some cool new features like a dot matrix display; so I decided to take a risk on it.
My Pathfinder has steering wheel controls and the crappy Bose system. Yes it sounds good when it works, but Bose stuff tends to break and is horrendously expensive to repair when it does, because every speaker has its own amplifier. OEM crap and Bose crap is expensive alone, so mixing the 2 is asking to be robbed.
Making the Bose speakers / amps happy
My speakers were still working, so I was going to keep them for $ savings. Of course Bose amps and speakers are finicky. The speakers are really low impedance 1-2 ohms, so you can't just bypass the amps and drive the speakers with an aftermarket headunit or you'll burn it out. If you wire your aftermarket speaker wires to the Bose amp inputs you will get horrible results.
If you wire your stereo with sub-4 volt RCA preamp outputs directly to the Bose inputs it is pretty much guaranteed the results will suck. It seems some people have wired their stereo with 4 volt RCA outputs directly to the Bose amps and it has worked well. The foolproof way to wire it up is to wire the stereo preamp outputs to an integration adapter with adjustable gain, and the adapter to the Bose inputs. I'm using an old Scosche FAI-3 integration adapter. They don't sell it anymore, but they sell a SLC-4 which is the same thing.
A final problem about 4 Bose amps and an electric antenna. Aftermarket stereos these days tend to have 1 lead designed to signal an amp to turn on or an electric antenna to extend. When you have just those 2 things, it generally has enough power to get the job done. But if you have 4 amps and possibly an antenna it is usually just too much. The answer is to install a relay that has enough power to turn all 4 or 5 devices on.
Some parts you'll need -
Metra 70-7551 Nissan 1995-2007 with RCA outputs or 70-7550 with bare wires
Metra xxxxx Nissan 1987-1994 with bare wires
Metra 40-NI10 Nissan antenna adapter to aftermarket stereo
Adapter for OEM steering wheel control of aftermarket stereo
PAC Audio makes adapters to control your new aftermarket stereo with your OEM steering wheel controls. Most car manufacturers have one wire that goes to the steering wheel remote control. Each button contains a resistor with a different ohm rating. The stereo detects the resistance on that single wire to determine which function is being requested.
So of course PAC adapters expect this, and of course Nissan (along with Harley) do something different. They've got one wire for each remote control button. That's pretty wasteful in the automotive industry, but whatever. For a PAC Audio adapter to work with a Nissan, you've got to connect a different resistor between 46 ohms and 2k ohms to each of the remote control button wires, then connect all of those resistors to the white wire on the PAC unit. Doing this right in the wires as they recommend is asking for a fatigue break, so I am going to mount them on a PCB and use stranded wire coming on and off the board.
One note of caution if you try to test the wires coming from the remote control buttons. They aren't just simple switches. With the ignition off, if you press a button, you will find the resistance changes slightly on all 5 wires. This confused the hell out of me until I decided to try it with the key in ACC. When I did that, the resistance changed a lot on only the wire connected to that button and not at all on the other 4 wires. So the remote control has some kind of probably solid state relay inside it that must be powered to work.
So, I decided to put the amp turn-on relay and steering wheel resistor network in a single little box. I used the following parts -
Radio Shack - Expensive but immediately available
275-233 SPST 12VDC Reed Relay with 0.5A@125VAC contacts
276-159 Dual General Purpose IC PC Board
270-408 Enclosed 2 AA Battery Holder
22 gauge stranded wire
I gutted the battery holder, enlarged the hole with the wires exiting and made a new hole on the other side so I could get a total of 10 wires out of it. I scored the PCB so it would fit, then I soldered the relay, resistors and wires to the PCB board. Finally I buttoned it all up and got ready to wire it in.
The wire crimpers sold at auto stores do a horrible job. They don't make a reliable and solid connection. You should use a ratcheting style terminal crimper. They can be found on eBay for about $30, but are generally $100 if you go to a good electrical hardware shop. You can learn a lot about proper crimping here -