Stock sub not working

My dad has a 98 Ford Expo and it has the mach sound system with a 8in sub in the rear...Well its really week with bass my bros 00 F-150 with 2 stock door speakers is better...I was messing around in the rear of the turck and looked and noticed that its not even moving(dosnt appear to be working)Is there any idea on why it wouldnt be working? Just currious because Iam going on a road trip in it and I want some bass...