Welcome to NICO!
You have the base system (i.e. not
Bose), correct? There's really not much to troubleshoot - as
PapaSmurf2k3 said above, it's more than likely a loose connection, perhaps a frayed wire or something. The easiest place to start would be to check each speaker's connections; see
this post for a guide on removing the interior door panel to gain access to the speakers.