hi,
I find out the back motor is controlled by a 10 pins LB1640 as seen on the this picture,

i tested the motor with a 9v battery and its ok, (the wire blue and white are the + and - of the motor)

so the problem is the LB1640 does not get what its need on IN1 or IN2

on the pics below i inverted the picture on the bottom to match the circuit on the top and try to find out the componants before IN1 and IN2 on the LB1640