Hello, I'm installing a 12" sub in my car to replace my 10. When I got it all wired up, the amp green light is on, but no power is going to sub. The settings on the heeadunit are correct, the amp is getting 12v on my multimeter, and the sub is wired correctly. One thing I noticed is if I put my multimeter to the + ' on the output going from amp to sub, theres 0 volts. Is that normal? What could be the issue?