anti lock light and brake light on 1992 2wd safari

[love safari]

hi. my 1992 safari 2wd anti lock and brake light is on.i went to mechanic to diagnose the problem he said the rear wheel driver side sensor is out .another mechanic test it and said that the front wheel passenger side sensor is out .i don't trust both of them because were i am coming from are baaaaaad mechanics for American cars which made me a butter mechanic than them on my safari van .what you think the problem is please????? if it sensor problem can you tell me how to locate it so i can clean it or replace it