I have a 95 LESC and spend a lot of time reading Previa forums. Here are some pointers that may be helpful.

A similar problem is discussed in the following thread:

http://autos.groups.yahoo.com/group/previa/message/30808

If you need the wiring diagrams, the Yahoo Previa group has the 96 FSM posted in the Files section which may or may not help with a 92.

http://autos.groups.yahoo.com/group/previa/files/96%20Previa%20Factory%20Service%20Manual/

Another good resource is the Previa Technical Discussion forum on http://toyotavans.net/forum/

I think others on here are right, it is a bad ground.

Good luck!