my guess is that its a "standard" double pole / double throw switch. Center contacts are the "inputs" (ie. power and ground), while one "pair" connects to "forward" the motor and the other "reverses" the motor.... the "polarity reversal" could be done either internal to the switch or down at the motor. Hence, you just need to look / check for continuity when the switch is pushed on way or the other... of course you do this with it disconnected.
further, its more likely that you have a wiring issue / connector issue. Have you check your voltages (AND grounds) at / near the switch and at / near the motor?