Problem with sub sound...maybe wiring

Hello,
The bazooka in my truck recently has started acting up. It's as if it's stuck on too high of volume, the volume button doesn't even work, but after a few minutes of me driving it will go back to normal and have a little crackling sound for a bit then it'll be fine. Usually happens in the morning when I first start the truck up. It is about 10 -11 years old, maybe just a loose wire? How can I test the harness? I have a meter just not sure what to look for. I did just have a new stereo put in 4-5 months ago and I put in new speakers. Thanks for any help!