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I went through the 68 wiring diagrams but could only find this one referencing wires 640 and 997. I did not find any other document showing what they are connected to.
756803
 

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Those two pins on top of a '68 brake light switch are connected together. I have a '67, but only the '68 switch is available as a replacement. So you have to rig up an adapter or cut the lead from a '68 harness. Anyone got a scrap '68 harness they can cut the brake warning switch lead from for me?
I might. I replaced by harness last summer. I think kept the old one for parts
 

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I went through the 68 wiring diagrams but could only find this one referencing wires 640 and 997. I did not find any other document showing what they are connected to. View attachment 756803
They are connected to the main (large) under-dash wiring harness. The "Belt" light does not appear in that diagram because it's an accessory.
 

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The seat belt light apparently gets turned on by the optional seat belt relay
That is for the round convenience control seat belt light. The one being discussed here is on the instrument panel. The data supported conclusion is that the seat belt indicator on the instrument panel was not used.
 

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They are connected to the main (large) under-dash wiring harness. The "Belt" light does not appear in that diagram because it's an accessory.
That I saw. I was looking for the detail about the connection to the switch itself. That I can't find on any of the diagrams. I wanted to determine if the second wire was a ground, as I had originally assumed, or if the second wire was the proof wire.
 

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I went through the 68 wiring diagrams but could only find this one referencing wires 640 and 997. I did not find any other document showing what they are connected to. View attachment 756803
Here is a summary of the wiring:
756960


997 is a typo. It is wire 977. 977A is the prove out wire to test the light when the ignition switch is in start position.
 

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That is for the round convenience control seat belt light. The one being discussed here is on the instrument panel. The data supported conclusion is that the seat belt indicator on the instrument panel was not used.
I have seen it used alright. But it had a pretty high failure rate. I'm thinking it was part of the deluxe seat belt option.
 

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That I saw. I was looking for the detail about the connection to the switch itself. That I can't find on any of the diagrams. I wanted to determine if the second wire was a ground, as I had originally assumed, or if the second wire was the proof wire.
Proof wire, tests during starter engagement. If you tear apart the nylon sensor, you'll see both wires connected at the sensor switch.
 

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Here is a summary of the wiring:
997 is a typo. It is wire 977. 977A is the prove out wire to test the light when the ignition switch is in start position.
Flade - a picture is worth a thousand words - thanks for the diagram, complete with wire color codes. John
 
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