Volvo 740 turbo having electrical ground problems. How does some of the components keep gound when others fail?

There are numerous ground points on a Volvo 740. The two most troublesome are located on the left and right inner front fenders - on the driver side is the ground for the coil and some lights. On the passenger side is the ground for the radio interference relay, which is really a ballast for the fuel injection system. The engine computers are grounded to lugs on the intake manifold. All are subject to corrosion. Believe or not, the windshield wiper ground is also troublesome, as evident when the wipers won't park or won't even shut off. I ran auxiliary ground wires, #12 stranded, from battery negative, to EACH grounding point. I also cleaned the ground terminals on the inner fenders and coated the connections with dialectric grease. Use a 12 volt probe, on the lights up - put the alligator clip on battery positive and probe the grounding points. If the light lights and is strong, ground is good. Weak light or no light, ground is questionable.

Some of the Volvo control circuits run on as little as 3.5 volts. A flaky ground can cause all kinds of mysterious problems.