Check the wiring harness for damaged or chaffed wires. If the harness appears to be in good condition, remove the main bulkhead connector from the transmission.
With the key off, use the diagram below as a guide to locate the correct pins at the transmission connector. Measure the resistance at the main transmission connector by probing the red lead to pin 7 (red wire) and the black lead to pin 9 (yellow wire)
Is the resistance between 3 and 8 Ohms?