Low level outputs from Ford head units are balanced differential signals, not unbalanced single ended. If you’re not familiar with these engineering terms, what this means is there is not a ground return through a shield on a coaxial cable that’s common to the chassis of the car. There should be a two wire pair with a separate shield for each channel, and both wires share signal independent from chassis ground. If you ground one of those wires, you will cause significant distortion of the signal running to the amplifier, which can also cause damage to the head unit, depending on its design. If you are using a conventional aftermarket amplifier, the differential signal pair needs to be converted to an unbalanced single ended coaxial signal which allows the return to be grounded to the shield. There are processing units available that do this, but most are fairly expensive. The other way to do it is to ensure the aftermarket amplifier accepts a true differential input.