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Hi, my name is Jay
The problem sounds like the transmission output speed sensor is not working correctly and will cause the speedometer to not work and the transmission to not shift and go into failure mode. If the light came on then there should be codes stored in the computer for the mechanic to check and pinpoint the problem. The sensor or wires may have been damaged when installing the transmission.
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What does not work on the dash? If just not getting speed then the transmission may have a defective speed sensor. This would cause the transmission to not go to overdrive and the speed to not work correctly. They may have damaged wiring to sensor or damaged sensor when installing.
This is a more serious problem that will need checked since there may be an electrical connection that is shorting the system out. I have not ran into this exact issue and it should be addressed in case it is wires shorting in dash and may be a fire hazard. This may be entire issue with dash assembly since Ford has have many car lines with electrical issues on connections to dash.