LATCH limits on 2002 Sentra?
You should be able to get a good install with the belt. Have you taken the car and seat to a CPST or a seat check to see if they can get it installed? I have a LATCH Manual in my car, I can look up the specifications for LATCH in the morning. Can you private message me a reminder? I will probably need one :)
Is it that the belt is not locking? Where are you trying ot install the seat and with what kind of belt?