I've replaced the alternator on my '98 4-cylinder Frontier, was pretty easy and straightforward.
Disconnect negative battery.
Remove the air hose that is over the drive belts.
Disconnect the wires/harness at the alternator
Loosen the bolt that secures the adjustment (bolt head faces forward).
Loosen the long bolt that serves as the alternator body pivot; you may need to access from underneath, so remove the plastic engine protection pan if you have one.
Loosen the long fully-threaded bolt that is the tension adjuster (bolt head faces right/passenger side).
Pull off the alternator belt, pull the alternator out.
Like they say: installation is the opposite !!!
If you need to replace the alternator belt (not a bad idea), the other belts must be removed first for access. I just replaced all my belts a month ago, two were 10 years old. Good idea to keep the old belts as emergency spares too.
If you're wondering: my replacement alternator (about 5 years ago) came from O'Reilly, and has a lifetime warranty.
