The cruise control was designed to operate only if the car is in gear, and is connected to the start inhibitor switch for this. The start inhibitor switch must be open for the cruise control to work. Your car is probably wired to start without being in Park or Neutral, you'll have to fix that.
You should find pink and blue wires connecting to the start inhibitor switch in the shift mechanism.
Connector D, which is near the brake light switch under the dashboard, has a pink jumper wire. This pink jumper wire has to be cut for the cruise control to work in cars which have been converted to manual transmission. See the image below. Post back if you have wiring questions.

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john
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