These reversible DC motors use 4 high current transistor switches. Two switches connect in series between + and - (call these A and B). Two more (call these C and D) also connect + and -. The motor is connected between the A/B connection and the C/D connection. Turn on switches A and D and the motor turns one way. Turn on B and C and the motor turns the other way. The 'brains' of the unit are what control the switches. If you're only seeing 6 volts across the motor, then the transistor switches are not turning on fully, perhaps due to low voltage to the 'brain'. I'd look and see if there's another fuse (small one for the brain) with a bad connection, or another bad connection feeding the 'brain'.