[Mr V]

If you only want direction control and running at full power is not a problem then the way to go is a pair of automotive relays of the 5 pin variety. They are available in a 40a rating, though there are also ones rated for 20 or 30 amps so be sure to check the specs before purchase.

The 5 pins are as follows.

Coil +
Coil -
Common (C)
Normally Closed (NC)
Normally Open (NO)

To wire the "power side"

Connect each of the motor's power leads to the C terminal of one of the relays. In other words the red wire goes to one relay common terminal and the black wire goes to the common wire of the other relay.

Connect the NC terminals of both relays to the negative side of your power source.

Connect the NO terminals of both of the relays to the positive side of your power source.

To wire the "control side"

Connect the coil (-) terminal of both relays to the negative side of your power source.

Connect each of the coil (+) terminal the relays to a separate switch, I'd go with a momentary contact style. It only needs to be able to safely switch 200ma at a nominal 12v. Then connect the other switch terminals to the + side of the power source.

Now operate one of the switches and it will energize the relay it is connected to which will supply power to the motor lead connected to it. The other relay not being energized will provide the path to the negative side of your power source. Operate the second switch and the other relay becomes the power source causing the motor to operate in the other direction.

The one problem with this set up is that it will operate like a motor controller in brake mode. This is because when both relays are are not energized the motor leads are connected together via the ground side wiring going to the battery.

The other caveat is that some relays have a clamping diode to prevent the back emf generated by cutting power from back feeding the device doing the switching. These are typically used when a transistor is used as the switching source. Other relays do not have the diode and those don't really have a coil (+) and coil (-) and thus do not have a polarity.

The most common 5 pin relay is usually referred to as a "Bosch relay" because it uses a form factor that they developed, even though Bosch is not in the relay business anymore and a number of manufactures make interchangeable relays.

The terminals on many Bosch style relays are marked with numbers which are as follows

85 Coil (-)
86 Coil (+)
30 Common
87 Normally Open
87a Normally Closed

All of that said the easier way to do this is with an extra motor controller and a device like Andy Mark's Thrifty Throttle. The Thrifty Throttle generates a PWM signal to control the motor controller so you get speed control as well as directional control.

Also be sure to provide circuit protection appropriate for the size of wire in your system. You can purchase an inline MAXI fuse holder that will allow you to use one of the 40a breakers for the PDP or an ATC/ATO fuse holder that will accept a 30a or lower breaker. The other option that you can find at the auto parts store is a self contained breaker that has a pair of #10 studs and nuts for terminals.