The wire problem is not unique to your old merc. It is the bane of Mercs of this vintage, and there is little one can do. Once the insulation has started to fail in one area, the entire harness can be expected to soon follow suit.

I have used a "
3M (tm) Scotchcast #82-A power Cable Splice Kit", and the old plug, and new generic 7-conductor cable 12gauge minimum (i used two lengths of 12/4 water resistant power cable made my Carol company, "P-7K 123033 MSHA)", but samey-same.

You may luck-out and find a good one on ebay, but even the NOS harnesses (if you could actually find one), would be on the verge of loosing thier insulation.

The Scotchcast kit is somewhat pricey, but nothing werks better that I know of.

Of course, you
could just hard-wire everything, if you have the skill and patience, and just elimminate the entire plug/socket do-hicky on the side.