best thing to do is make sure there is a very good earth point from battery to body, needs to be a big connection such as an M8 (13mm spanner size normanlly) bolt connection. i welded a nut to the bulkhead in front of the battery on VJM. this was before paint. The area where the terminal actually touched the body didn't need to be paint free, helps if you don't scrape the paint off actually as that just encourages corrosion which negates the connection.
i made sure the threads inside the now captive nut were perfectly clean, a run through with a tap before putting in the bolt was enough. the bolt then makes a superb connection to the nut, just imagine how much surface area there is touching inside a nut & bolt? far more than a cleaned off area of bodywork would give.
you can then earth any component with very short runs direct to the body in the same way.
this was proven to be the case with my headlights.
i welded in 2 M6 bolts to the inner wing directly under the lamps, these actually doubled up as mounts for my horns..
again these were painted over but the threads kept clean.
when it came to my headlight wiring i made very short earth return wires, can't have been more than 4" long. i also made individual ones for each lamp..
together with making sure the supply wires were free from resistance by letting in new wire from the old stuff and soldering connections not just crimping them on my headlights wroked perfectly, easily as good as modern cars even with only the standard bulbs fitted.
so the moral is, you can never have too many earths but just make sure they are all good ones
p.s. i'm reasonably sure the sud wiper motor earths through the bracket that mounts it to the body