It is under the dash, to the left of the steering wheel, and right where the plastic dash stops. https://www.google.com/search?q=2004+nissan+frontier+obd+location&rlz=1C1CHBF_enUS813US819&sxsrf=ACYBGNRnd7wzxHdyz8qpcKn4Bxau3i4XLQ:1571110108150&tbm=isch&source=iu&ictx=1&fir=wEoswCxeAIPBVM%253A%252CE6pxZeCXtKM3-M%252C_&vet=1&usg=AI4_-kT-kM5L3r83vfWkqdkDVhVEgP6t4g&sa=X&ved=2ahUKEwiTxJifqZ3lAhUrgK0KHZjlCHgQ9QEwAHoECAYQBA#imgrc=wEoswCxeAIPBVM:&vet=1
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It is under the dash, to the left of the steering wheel, and right where the plastic dash stops.
https://www.google.com/search?q=2004+nissan+frontier+obd+location&rlz=1C1CHBF_enUS813US819&sxsrf=ACYBGNRnd7wzxHdyz8qpcKn4Bxau3i4XLQ:1571110108150&tbm=isch&source=iu&ictx=1&fir=wEoswCxeAIPBVM%253A%252CE6pxZeCXtKM3-M%252C_&vet=1&usg=AI4_-kT-kM5L3r83vfWkqdkDVhVEgP6t4g&sa=X&ved=2ahUKEwiTxJifqZ3lAhUrgK0KHZjlCHgQ9QEwAHoECAYQBA#imgrc=wEoswCxeAIPBVM:&vet=1