There is no specific wiring diagram from the 1986 +4 on the GoMog pages.
I recommend that you try to pick a "close-pick" by starting to look at you fuse box.

If you have a standard Lucas 7FJ fuse box then then your loom will be close to the 1984 diagram

Otherwise the 1987 +4 diagram might be most likely better.

Hope this helps
Tim

Tim
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1984 Morgan 4/4 fourseater