coil? square box on fender? help

so finishing up the rebuild, the bronc was driving down the road like a champ!! however, no taillights or brake lights. I work on the wiring and amazing I have tailights and brake lights. I then go to start up the bronco and take it down the road with all the lights and everything. will not start, is not even firing. ive checked all my fuses and all the plugs I unpluged (cuz i had some issues and I unplugged all the plugs from under the hood) but it will not start!! could the coil have just gone out, when it has started fine for the last 30 starts? what about the square box on the side of the fender. When I check power with a test light, there is power coming out of the box, but it seems dim. also the other plug that goes over to the coil, the power is intermittent (flashing on the light). I know that the stuff I did to the tailights is on a different circuit and all the fuses are good. HElP...