So, nobody feels the Simple Green will eat away at the insulation over time? This proceedure sure sounds less "aggressive" than running the harness in a washing machine Not knocking that idea, like a lot of things...If it WORKS and it makes life easier, what the hell!
Any chemical with solvents or petroleum components has the POTENTIAL to affect plastics and rubber. Having said that, there are a number of counter arguments:
1) A quick wipe of the surface is not going to affect the material as much as soaking would.
2) Anything that doesn't wipe off immediately will probably evaporate quickly.
3) The crud you're cleaning off may contain gasoline & petroleum compounds, so removing those is better than leaving them on.
4) You're cleaning insulation that's suffered the ravages of 40-50 years of aging. One gentle surface cleaning is probably not going to do much harm compared to that.
The MSDS for Simple Green shows the following:
Water - as you might expect, not a concern unless it gets into the connectors and inside the insulation, where it could lead to corrosion if not fully dried
2‐butoxyethanol - surfactant (soap, basically), should be compatible with most plastics
Ethoxylated Alcohol - surfactant, should not be a problem
Tetrapotassium Pyrophosphate - an emulsifer (keeps removed material suspended in the cleaning solution). Sounds nasty but should not be a problem
Sodium Citrate - emulsifier, mild and should not be a problem
Fragrance & Colorant - unlikely to be incompatible, and would only be a very small percentage.
So I think you'll be OK with Simple Green, but like with any other chemical it's best to try it out on a test piece first. The MSDS for Dawn dishwashing liquid has a list of similarly ominous-sounding compounds, but would also be unlikely to harm the insulation.
The washing machine method is clever, but I would be concerned about mechanically damaging the harness, and more so about getting it fully dried, especially inside the connectors. Also, Oxyclean contains hydrogen peroxide which does pose a problem for some plastics. I'm not sure if the insulation is PVC (commonly used nowadays, don't know if it was in the 60s), but that should be OK with hydrogen peroxide. Other materials in the harness may or may not be.