Grounded to the same place? Noise traveling through a vehicle is typically caused by a difference in grounding points or non-sufficient ground. Therefore, feedback is found all over the place to compensate for poor choice in ground.

#1) Increase the surface area of contact the current ground maintains. You can't increase the need without increasing the ability to transfer that need.

or

#2) Find a second location for a ground and hope a ground loop isn't created.