It really depends on how how much you want to spend.
The most inexpensive way is to integrate with the factory amplifier.
The easiest way is to get an aux in adapter for your factory radio.
Here is the wiring diagram for your 2001 Nissan Pathfinder with the Bose system.
2001NissanPathfinderBoseRadio.pdf
GiODi was correct, the amplifiers for the front speakers are located on the speakers themselves. The amplifier for the rear speakers is separate.
The wires that are important to you are all located at the headunit connection in the dashboard. No rewiring or taking off door panels is necessary. Although you may want to purchase a wiring harness adapter to make the connections easier.
COMPUTER ---> RADIO CONNECTOR
Left + ---> Pin2 (Light Green) and Pin14 (Pink)
Right + ---> Pin4 (Blue/Green) and Pin16 (Light Green)
Audio Ground ---> Pin1 (Purple) and Pin3 (Blue/Red)
Audio Ground ---> Pin13 (Blue) and Pin15 (Purple)
Remote Out* ---> Pin12 (Light Green/Red)
Case Ground* ---> Pin11
In this case I have assumed that you are only using a stereo signal from your computer (just left and right signals). If you have a multichannel card with rear outputs you can send separate signals to each.
*The Remote out in this case is used to turn on the factory amplifiers. Also, if you plan on using the your antenna, you may need to connect to Pin5 (Red) - this appears to be for an antenna amplifier. You can get this signal from a DC-DC power supply, or if that is not an option you can simply use the ACC signal from your car. Note: if you use the ACC signal, the amplifiers will be on as soon as the key is turned, which may result in a "thump" or "pop" sound when the computer boots. You should use a meter to check and make sure that this line is indeed 12 volts and not something else - especially as the voltage is not mentioned in the wiring diagram.
*Pin 11 appears to be a shield for the audio lines to the rear of the car and the rear amplifier. It is unclear whether it is required that you apply a ground here or not. If you have any noise problems you may decide to try grounding your case here.
Disclaimer:
I have not done an install in this particular model before. These are suggestions to help you with your project. I cannot guarantee that this information is correct, this is only what I can recommend after looking at the radio wiring diagram.
Good Luck!