As you can see in the schematic the 12V battery (or use other voltages) is connected with the plus at terminal T1 and minus at terminal T2. The contact 1 and 4 are connected together as 2 and 3 are ...

With these mods, this circuit will last quite a long time, especially if you make things easier for the relay by turning off power to the motor before switching directions. It is simple and effective ...