74 Duster Electric problems

My duster has been having electric problems since i picked it up in January. It started off with small things. then the tail and brake lights started blowing fuses ever once in a while then completely stoped working. During that time the only work i did was change the radio that the previous owner had added. Any time i turn on the headlights the fuse blows and the tailights stop working and even with a good fuse and the headlights off the brake lights dont work. I did find that one of the tailight sockets was bad so i replaced it. I have pulled out the instrement cluster and started to pull the underdash wiring harness down to try to find a short but to no result yet. I am wondering if i should just try and get another harness and see if that helps or if it would be easier to just fix the one i got. I have no testers or very many tools so it makes it kind of hard, this is my first project i love the car just not the electric stuff its a pain. Any help would be great.