Hi, I have struck this problem before being a locksmith, it could be other problems but I think you may find that the most common problem is that the key is cut too deep. The key should be cut back to factory specs and it should get rid of the problem. That would be the 1st thing I try. Another thing that can go wrong is on alot of models I think starting around that time, Toyota started putting what is called split wafers in the locks. The reason for this is if an object such as a screw driver or wrong key is put in, it is meant to throw the split wafers out of alignment causing the lock to be securely locked, but disabled. If that is the case the only thing you can do is take it to a locksmith to get the split wafers realigned or taken out ll together....