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Hi, new to the forum and in need of help please.
I'm building a tamiya midnight pumpkin, all was going well till I put on the rear wheels and found that when I manually turn one wheel the opposite wheel turns the other direction.
I've revisited the gearbox a number of times but cannot seem to find the fault.
Your advice would be greatly appreciated.

Congratulations, you've correctly assrmbled your pumpkin.

All jokes aside that is exactly what should happen due to the differential in the gearbox.

Really! Wow
The wife was having a real laugh at me.
So what happens when I power up the motor, will it run with the wheels going opposite to each other or will it just sort itself out?

Correct. Everything will be good. If you want to check it for giggles. If you hold the opposite wheel when turning one. You will then turn the motor.

When just spinning one wheel the motor magnets are providing resistance so you are just spinning the spider gears in the differential. When you spin one forward the spider gears cause the other wheel to spin the opposite way. The spider gears allow the outside wheel to spin faster while going around corners.

Thank you, I understand now, thought I was going nuts!

The same principle applies to full size vehicle axles as well. This is known as an open differential. A locked differential means that both wheels receive equal power. This is achieved by using what's known as a locker or having a posi unit installed.

So your saying, if I have some popeye spinnich and with one arm lift up the rear end of my transit van, and the other hand I spin a wheel, the opposite wheel will rotate the opposite direction,
I never knew that! I'm impressed!

Yup, that's what would happen.