How does this connector come apart? (1971 Duster)

I noticed the empty position in the plug with a brown stain. To my recollection - I’ve never handled this connector before and I was assuming that this is just an empty slot that has never been used.
Now that you mention it…there is a heavy red wire in the same area that has a weird splice and is wrapped with tape. Maybe - many years ago - the red wire pulled out of the connector and I (or someone at a shop) spliced it together.

I’ve had this car for 52 years now and I’m finding that I don’t remember everything that I did to the car. I didn’t have much money in my younger days and I sometimes did what I could at no cost to keep things working.