I ran a 1 AWG cable from the negative terminal of the battery to a chassis body bolt in the engine compartment. I then disconnected the short cable from the battery to the chasis ground near the tailgate. The cable i installed ran under the rugs next to a plastic channel carrying the positive cable to the battery. It went through a hole i drilled in an unused port in the firewall near the brake pedal. By forcing the battery return current to flow next to the current in the positive battery cable, the resulting magnetic fields would be reduced.
Before i put in my cable, the largest magnetic fields at idle were near the driver's left foot with values about 210 milliGauss (or 21 microTesla). Those numbers are consistent with those published in a Volvo monograph. After the cable was installed, the field dropped to about 30 milliGauss. For curiosity, i then laid a 1/4" steel plate over the carpet on the driver's floor. That resulted in a maximum field of about 16 milliGauss.
I don't have any reason to claim that the time-varying magnetic fields are harmful. Maybe they are even beneficial. I felt i'd rather not have that souce of magnetic "noise" in the car. I also wanted to use this experiment to compare with theory in my course on electromagnetics. My magnetometer was a "60Hz magnetometer" which i believe is a broadband instrument, measuring all but the DC (static) magnetic fields. Despite the fact that there are diodes trying to smooth out the three-phase AC from the alternator, the time-varying fields measured are due to time-varying currents due to the voltage regulator switching on and off.