Need help with fuse that keeps blowing

I have a '71 Swinger and the fuse that runs the dash panel light, gear shift light and tail lights keeps blowing. When I put in a new fuse everything works for a while. However, when I turn on the headlights, both green turn signal lights in the panel light up (they don't blink - they just light up). When I turn on the right or left signal, the aftermarket radio (installed by a previous owner) hesitates for a second and then starts playing again, and the turn signal clicks, but very slowly. Shortly after, the fuse blows again. Weird stuff going on. I'm guessing the problem stems from the person who installed the stereo, but not sure what to look for to fix it???