Pioneer VSX-D498

Perhaps the unit isn't switched to 5.1

Maybe the movie wasn't recorded in 5.1 or if it was perhaps the recording was faulty. Try a known good recording to check the system.

Check your outputs and inputs plus your speaker cables.

Check the fuse if there's a seperate one for left and right, which there should be.

Try swapping the front speakers to the rear and the rear to the front and see if the fault is still with the rear sound.

BMW

Use the speaker test functionality on your remote to check all your speakers are in properly.

You may also need to hit the surround mode or input mode button to get it working.