If you suspect wiring harness problems, well, you're probably right.
1986 is a prime year to have the problems, and by now that lousy original insulation is way past its prime. If you have harness problems, the only real fix is a replacement. The harness from an 87 or 88 will fit with minimal work. You may have to change the battery cables along with it in order to get everything to work right. The cables switched sides, + for -, in 1987.

If you have the problem, you'll see wiring harness rot, i.e. bare copper wire, corroded copper, crumbling bits of plastic insulation on individual strands, at the following locations:

) Grey 8-pin connector at firewall
) starter solenoid
) Under intake manifold
) behind the alternator and at the oil sender
) under the front pulleys

Some wires can be patched; the small alternator red wire and the oil sender wire can be re-routed successfully around the right side of the engine compartment. It's not that hard to do to cut out a small bad section like that; but wires that have to go through the main harness are going to prove very difficult, probably impossible, to isolate and repair in any kind of satisfactory way. If there's extensive damage, it's time for a harness. Good news is, even many junkyard cars have had new harnesses installed at some point. They can be pulled out and transplanted fairly easily, for much less cost than a new one.

Good luck with it!
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::: Rob Bareiss, New London CT ::: 92 244 M47 211K ::: 90 745GL M47 273K ::: 88 245DL AW70 190K ::: 84 242DL Project ::: 70 VW Bus ::: 70 VW Pickup Project ::: 71 VW Notchback :::