The splice is a bit dodgy but are you familiar with Sikaflex? I put about A 4" length over one end before I soldered them, then I taped them generously and slid the tubing over it, and shoved Sikaflex in one end til it squeezed out the other. I'd be real surprised if moisture got through even if submerged 24/7 but just in the air, that may as well be a factory seal.

I will do a current test on the ground wire though, that could be instructive.

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1986 300SDL, 362K
1984 300D, 138K