From what I understand you take the top off the dashboard, starting woth both doors open and doing something with the binnacle you peel the top of the dash off to get the radio out, the heater is then in there too.
Mine sometimes stops blowing, the motor is fine it's just the s****y Nissan wiring that keeps playing up, Jer-Jer-Jer-Jiggle the wires and it's ok again. It's up the top of the dash though I can tell you that.