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Make sure the solder tab and wire on the roller assembly does not touch the top of the truck frame when the roller is depressed. There is very little clearance in this area when the roller is depressed, and it is easy for the wire to short to the frame. Put a piece of electrical tape on the truck frame above the roller pickup assembly if in doubt.
If this does not solve the problem, disconnect the power wire from the roller where it connects to the mechanism inside the car, to isolate the roller from the mechanism. If the short remains, there is a problem with the roller and wiring as stated above. If the short is now gone, the problem may be a short in the mechanism somewhere. Visually trace the wiring, and replace any old, **** wire.

If you have any other powered car, whether lighted or operating, the roller pickup assemblies on similar type trucks are pretty much the same.

Larry