1996 Nissian Pathfinder

I think there's a factory amp in these vehicles (even in the ones that don't have the bose system) but usually it just hooks up like a regular stereo, with the exception being you have to make sure the factory amp turn-on wire is hooked up properly.

If it had an aftermarket pioneer head unit already installed and the pioneer worked fine, just use the same wire colors of the alpine that were used with the pioneer.  As long as both units were manufactured in the last 6 or 7 years they should both follow the same EIA wire color standard.

As for the vehicle's wires itself, here's what we have for the wiring info and you can also check out this diagram but I you really should only need either of these things as a last resort.

If none of this helps make sure there are no speaker wires that are shorted out or touching bare metal in the vehicle.  Such a condition could cause the head unit's internal amplifier to go into protection mode.  When in protection mode it appears to be functioning normally, except there will be no sound output until that speaker short condition is eliminated.