How to auotmatically reverse a 12v motor on successive activations
The Finder 20 Series are step relays with plenty of power for your application but they only have two contacts.







simulate this circuit &ndash; Schematic created using CircuitLab

A DPDT (double-pole, double-throw) impulse relay should do what you need. 


The sketch shows the relay with V+ connected to the left side of the motor.
The next time the 12V turns on the relay will change state (managed by some internal contacts not shown on the diagram). This will cause the motor to run the other direction.
And so on ...