Polarity reversing
You are right about needing relays to cope with the ampage.Simple reversing switches will burn out.It's quite simple to reverse a DC motor with 2 CO Relays.
The relays are 5 pin,marked-30,85,86,87,&87a .if you look at the picture on the
relay casing,you will see see that 87a is ON, & 87 is OFF, when the relay is not energised.if you look at my pdf,you will see that both motor terminals are connected to ground when off,when ONE of the relays is energised,the motor
will run.& by energising the other relay,the motor will run in the opposite
direction.The best source of suitable relays is your local breakers yard, relays rated at 16 Amps will be more than adequate for the job,
Good luck Terry.