You might try shielded spark plug cables. Or, move the lip mounted antenna as far away from the engine compartment as possible and assure you’ve got a good ground with those screws that secure the antenna mount to the lip. Those screws tighten up against the metal lip and should be allowed to puncture the paint and make a good ground contact. I’ve seen some installs that used small metal plates which are placed between the lip and the screws. My take on that is the antenna being used may be one that is “ground independent” meaning it doesn’t need a ground. These ground independent antennas are used when mounting an antenna to a non-conductive material such as fiberglass or the electric fold over antenna mounts.
Mobile antennas consist of two basic parts, one half is the vertical radiating element and the other equally important part is the ground-plane. That’s the roof or fender of the vehicle. Ideally, the antenna should be mounted as close to the center of the vehicle as practically possible. All other mounting locations come with trade-offs. But not everyone wants to pinch a hole in the middle of their roof, so they live with the less effective mounting methods. And there’s nothing wrong with that because it’s a personal preference.
