If there's a harness in the dash still, do not cut it off! The easiest thing to do is get a wiring harness that will connect to your Boss radio and then plug right in. You can get them from Amazon
or your local electronics store like Best Buy. You can sometimes find them at WalMart as well. The benefit of going to a place like Best Buy is that they have an installation bay with trained techs so they can ensure that you have the right harness. The colors on this harness should match that of the Boss stereo and it's just a matter of soldering/crimping them together.
If the harness has already been cut, its a little more difficult cause the factory wiring doesn't match what's on the Boss. I found the wiring colors for a 94 Pathfinder here
but couldn't find any for a 94 Nissan Truck. They should be the same/similar but either way your should still check the ignition/switched and constant power wires with a multimeter before connecting them to the new radio.
For the constant, you can get ground from any piece of metal inside the dash, touch it to the wire the suggested in the diagram. The meter should show 12v or just under. Do the same for the ignition/switched, you shouldn't get any reading. Turn on the ignition and it should go to 12v or just under.
For the speakers you can check the pair with a 9v battery. Take the two wires for lets say the left speaker, blue/white and blue yellow, and touch them to the positive and negative terminals of the battery. You should hear a pop from the speaker. If you don't most likely the wiring is different from what I have listed below which is fine.
After figuring out the which wires are the powers and ground you can basically take one wire and put it on the positive terminal of your 9v battery. Now go through the rest of the wires and tap them one at a time to the negative terminal. Do this until you hear a pop not the wire colors and which speaker made the noise. Now move on to another wire and repeat.
The downfall of doing this is that it doesn't tell you polarity. After you finishing connecting the radio to the in-dash wiring turn it on. Listen to the sound carefully. If it sounds tinny one or more of the speakers polarity are incorrect. Try to determine which speaker/s sound tinny and reverse there wiring. Check again and hopefully everything's sounding good.1994 Nissan Pathfinder Car Stereo Radio Wiring Diagram
Car Radio Constant 12V+ Wire: Red/Black
Car Radio Switched 12V+ Wire: Blue
Car Radio Ground Wire: Chassis
Car Radio Illumination Wire: Red/Yellow
Car Radio Dimmer Wire: N/A
Car Radio Antenna Trigger: N/A
Car Radio Amp Trigger Wire: N/A
Front Speakers Size: 5? x 7? Speakers
Front Speakers Location: Doors
Left Front Speaker Wire (+): Blue/White
Left Front Speaker Wire (-): Blue/Yellow
Right Front Speaker Wire (+): Brown
Right Front Speaker Wire (-): Brown/White
Rear Speakers Size: 5? x 7? Speakers
Rear Speakers Location: Roof Mount
Left Rear Speaker Wire (+): Red
Left Rear Speaker Wire (-): Green
Right Rear Speaker Wire (+): Blue
Right Rear Speaker Wire (-): Pink
I hope this was helpful