This two motor robot was initially published in this article many years ago:

https://www.instructables.com/id/Two-Motor-Robot/

I contacted the author of this Instructables and purchased this robot:

You can see on the photo that I made a few minor modifications. The white wire is connected to Terminal 1 of the left motor and the red wire is connected to Terminal 1 of the right motor. Terminal 2 of each of the two motors are both connected to the yellow wire.

I used a remote control from an old 1990s toy car and attached it to this robot. You can see the outcome in this video:

The same author published another article about one the two robots on Hack A Day:

https://hackaday.io/page/7307-drawing-robot

Step 1: Attaching the Wires

I connected the robot to an old ear phone cable. Then I soldered the three wires from the earphone cable/motor to remote control switch:

When the box is closed the red level half circle is pressing on both red and white terminals/soldered joints. The top fixture (the one with the screw) is connected to positive battery terminal. Triggering of the red lever (shown in the video) switches between proving power to left or right motor. The yellow cable is connected to negative terminal of the two 1.5 V AA battery power supply.

I also inserted a 3 cm stripped copper cable piece to connect the two 1.5 V AA batteries together:

The fixed remote control circuit is show below:

The red lever controls were either power is supplied to the left or right motor.

The circuit above was drawn with: https://easyeda.com online circuit drawing and PCB design software.

Step 2: Switch Contacts Fixing

The switch was initially not working properly and needed modifications.

The top copper fixture was conducting/providing current to both fixtures at the bottom and thus needed be be raised to stop both motors from turning ON when the red lever is set in mid position.

Trial 1:

Because I did not have a washer. I tried using a nut as shown in the photo below;

This was a wrong decision. The nut was very thick and none of the two motors were turning ON.

Trial 2:

I found a small piece of metal fixture in my inventory with a hole in it.

I cut a bit off with pliers so that it had the same shape as a washer:

This modification proved to work. The following video shows that the two motors are never both ON at the same time:

Step 3: Circuit 2

I modified the remote control to allow speed changing of the motor:

This circuit above could have been the original remote control circuit. I do not remember. On the left side of the remote control you can see a hole that was used for the missing red knob to switch between different speeds of the toy car.

Implementation of Circuit 2:

The brown cable is connect to Terminal 2 of both motors. 

Connecting the brown cable to white wire will provide the motors with 1.5 V supply and connecting the brown cable to yellow wire will provide the motors with 3 V supply, thus increasing the robot movement speed.

Step 3: Circuit 3

I also came up with a third circuit that can be used to switch the directions of the motors with just one switch (Motor3).

This circuit also allows turning ON of just one motor per switch setting (Motor 1 and Motor 2).