My toddler got a Roleplay 6V VW bus for Christmas from his Grandma and Grandpa. There are lots of VW busses in the family now, but I think this is the smallest. And the easiest to get stuck.
After more or less following him around the yard pushing him out of holes (and noticing all the holes he was burning in the ground from spinning his one tire) I got to investigating. I started thinking about how to fabricate an axle system to drive both wheels. How to attach it to the plastic body, etc. When I finally flipped it over and took a look I found the most glorious thing: the body is symmetrical!
This means that the gearbox assembly can be installed on either-- or both-- sides. Roleplay even offers replacement parts and a service manual on their website. So I ordered a replacement gearbox, GEAR001. There wasn't any confirmation anything was happening, but four days later it showed up on my doorstep.
Installation is easy. Simply remove the left rear wheel hubcap and cotter pin and slide the wheel off. The spacer is no longer used, so it can come off too. Then slip the second gearbox assembly over the axle into place. The one missing part is a single screw to hold the assembly in place, which I robbed from the circuit board inside (it has to come out for an upgrade I have in mind anyway). Then the tire, pin, and hubcap go back on.
Because the second motor that you just installed is rotated 180°, it would push backwards if simply plugged in. To fix this, simply reverse the polarity on the wires: splice the second motor's blue wire into the original motor's white wire, and the second motor's white wire into the original's blue. Cover with some heat-shrinking or electrical tape and you're good to go.
The bus is a beast. It will climb over gravel bumps and plow over patio chairs. He has also certainly driven full-force into a tree, which he found very funny. The original bus, on the terrain of our backyard, was beginning to lack excitement. The new bus is on the edge of his ability to control, and so he needs supervision. As he gets older, though, I am confident this modification will make the bus hold interest and be more useful much longer.