I think that you will find something is fried.
I would take a look at the caps first and see if there is any signs of burning on them.
Also take the speedo out car and fully look at it for signs of burning or possible shorts.
Have you tried unplugging the motor then plugging the batterys in ?
If you have everything wired right and the battery plugged in the right way round then its got to be the speedo thats gone.
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