I think I'd start by removing the relays and checking the electrical resistance from each motor terminal to ground and B+. With the relays removed, there shouldn't be any electrical path to ground or battery voltage, so if there is then there's a short in the circuit between the relays and the motors.
Also, the circuit is designed so that when both relays engage at the same time, then both sides of the motor are powered with B+, which is effectively the same as no power since the motor operates on the voltage difference (which drives the electrical current) across it's terminals. Vmotor = Vpnk/org - Vpnk/blk; Vmotor = 0 - 0 V = 0 V or Vmotor = 12 V - 12 V = 0 V, which gives the same drive for either condition.
Also, notice that all the motors are wired in parallel. Does this only occur in the rear motor, or do all three (or four) seem to be cycling?