Anyone know how to read Nissan ECU codes?

My wife's 02' Nissan Exterra has a couple of issues. First, the brake light keeps coming on. Second, now the check engine light keeps coming on. She took it to Autozone the last two times it came on and they said they are not getting any codes. They reset it and it comes back on after a month or so. I'm getting concerned. Should I take it to Nissan and pay $80 (rediculous) for a diagnostic, or is there a way to read the codes? 2002 Nissan Exterra

Thanks