Gear indicator on dash doesn't match the gear the shifter is in
 

My car, 2000 chevy blazer 4 X 4 LT, has this problem since I bought it. It works very well, I mean mechanically, the problem is only with the indicator that is shifted one step. In parking, it shows P but, when I select R, it shows anything in dash, it shows R when I select N, and shows N when I select D and so on. In other hand, it turns on the white light of R on the back, when I select N. It is a problem because I can have an accident. Where is the device that give the signal to dash board indicator? is there any wiring diagram? Thank you in advance.