Headrest grounding

What is most important is that all of your equipment is operating at the same voltage. Take a digital multimeter and read the DC voltage at your car's battery, while the car is running. Let's say it reads 13.4 Volts. Now take the same readings on all of your equipment. ie.. radio,amps,dvd player, tv screens, eq's, preamp, extra batteries, caps, etc.   Do they all read the same exact voltage within 0.1 Volts?  If they are all the same then there is no ground problems you may have defective and/or crappy product. If one or more reads a different voltage then make it the same as the rest of the equipment.  Sometimes devices that retrieve its power via the fuse box might be at a lower voltage than the battery. Maybe by only 0.5 Volts. But that is enough to cause engine noise, clicks,pops, ghosting, etc.   How do you make it the same? Simple go directly to the battery for power and ground with appropriate sized inline fuses.