reverse does not work
 

Hi, i have 2006 Mazda MPV (called Mazda8 here in Hong Kong) and there is something wrong with the automatic gearbox: it does not reverse in R, it actually goes forward. Same in N, it goes forward.
The dashboard shows the correct P R N D M according to the shift lever position.
The local shop wants to replace the entire transmission, i'm not sure how much they have checked. If anybody has some suggestions on how to find out what part is probably faulty, that would be appreciated.
Thanks in advance.