When I replaced my speakers I found one of them had a corroded connector, cleaned it and added dielectric grease. Problem solved.
It can also be the door wiring in the door jamb.
With the door open and music playing grab that mass of wires in that rubber tube in the door jamb and give them a wiggle and see if the speaker cuts in and out.
It can also be the door wiring in the door jamb.
With the door open and music playing grab that mass of wires in that rubber tube in the door jamb and give them a wiggle and see if the speaker cuts in and out.