Negative battery to engine
Engine to body
Negative battery to body
?
I do not suggest vaseline anywhere near any electrical connector.
My father put it on the battery posts once to try to keep corrosion from forming... it did that well ... but insulated the post from the battery cable...

Here is a problem I encountered years a go when working at a local School District.

I kept getting cars that had multiple bulbs burned out including the dash light bulbs. (By the way I hate electrical problems.)
I could not understand what was happening as my experience with my own cars over 25 years was that in that time I replaced 2 dash bulb lights and maybe 6 tail/brake lights on the various vehicles I owned during that time.
Theses Cars had several Dash Light Bulbs burned as well as 1-3 Tail Lights/Clearance Lights/Brake Lights burned out.
The 1 clue that I got did not register with me at the time. One of the Turn Signal Lights mounted low under a bumper of one of the cars had water in it.
Later when I was working for another employer who had a contract with one of the local Oil Companies my Boss mentioned they were having the same problem with many of the Oil Company cars.
What was causing the problem was that the drivers of the Company cars being required to keep the Cars clean were hosing off the Cars (sometimes several times a day) and when the directed the hose at the various lights on the car water was getting inside of the circuits of the light causing shorts or increased resistance resulting in a bunch of burned bulbs.
By Boss told them to stop directing the hose at the lights; which they did and the Bulb replacement problem returned to a normal level. And, saved the Oil Company a little money on repairs.

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84 300D, 82 Volvo 244Gl Diesel