Further explanation now that I have a little typing time:
This is the tamper detector in case someone attempts to use a slide hammer or similar destructive device to gain access to your trunk without the key.....usually a criminal.
There'* a small tab on the outside of the lock cylinder that gets released when this happens, the cable lets loose, grounding the single wire connection and triggering the factory alarm IF the doors are locked when it fails.
If it fails while driving or the car is not locked by the power locks or key fob, it sets the security light to indicate a fault in the system.
If you disconnect it, and the light goes out, this is your cause for the light. Leave it that way. There'* nothing lost. The metal tab on the lock cylinder is small and fragile enough that it has a limited lifetime. It was a cool idea, but could have been a bit beefier. This is the most common cause for a solid (non-flashing) security light. If this doesn't solve your problem, we'll look at it in more detail.
Yes, this happened to my 93 a LONG time ago, and I played H E L L finding the culprit. Since then, I've seen half a dozen cars here with the same problem/solution.