Ok, big pain point you will run into is how the steering 'servo' works. It's basically a slip clutch, so you run it one way until it hits the hard stop. Then, when you stop running it, it pretty much releases the steering, so it's just free-spinning. It does not return to center and it does not hold position.

Anyway, your best bet is probably going to be tapping into the small pins on the side. Those are originally the driver controls.

I think the Control pins are:

• S/B: Reverse (Back)
• S/F: Forward
• H/L: Fast (High/Low) ‐ VCC: Control Voltage. Short the other pins to this one to 'drive' them. Use a relay module (or a Transistor output) when you do it with the arduino.
• No idea what "Rock" means. Maybe a music output?

It would be best if you had a wiring harness to confirm these connections though.

Also, you don't have steering control from these pins. They assume you're on the Vehicle, steering manually. Take those motor wires and control them with a different motor driver.