Engine interference?

[Pedro_The_Swift]

Ever since we replaced the head unit we get a funny interference from (only) the drivers side A pillar speaker. Doesnt happen all the time and may not happen for a week,, The sound rises and falls with engine revs. If we get a mobile/bluetooth call during one of these interferences the voice only comes out of the other A pillar speaker.
Any ideas?

[Judo]

Speaker negative shorting to body earth?