68 valiant signet

Not familiar with Exploder axels so I'll assume the rotor is a hat type, e-brake shoes inside. Make sure all shoe components are good and adjusted so there's a slight "even" drag on both front & back shoe, backing plate shoe pedestals aren't grooved and a bit of white grease on flats. If they are grooved and not deeply you can flatten them, if deeply grooved build up with welds and level flat. Once you have even shoe grab back off star adjuster till you "just" stop hearing drag. Have someone pull cable directly from end of sheath = not at handle, one side at a time and note how much length needs to come out for engagement. Let's say an inch. Then, with handle cable at its cable sheath end, not connected to rear cables, pull out on cable under car and have someone slowly pull handle out and while you're pulling have them release handle. Then note how much cable length goes into sheath when they pull it in, let's say 2 inches. So with these things done you've established; 1- length of cable that needs to be pulled from each rear wheel to engage brake and 2 - length of pull "in" at handle cable end. Obviously you need to join rear cables with a cable end coupler (bulb ends of rear cables slide into center slot and become one loop to both rears), then a J-hook of sorts needs to be placed on handle cable end, this needs to have a length of threaded rod so you can give adjustment for tension/slack. Welding cables is not only difficult but sketchy, find "crimp" cable end stops or better if you can locate end clamps that have 2 screws, these provide great test fit because you can loosen and move on cable and tighten back up. Hard to explain these things, but I'll try...draw a circle with a line thru the center, then a small diamond dead center of the line, then a screw hole at each end. The circle splits, cable goes in diamond, screws secure cable in diamond. Wish I knew what these things are called... Once you get everything set and working wrap tightly the area of excess cable where you need to cut for length and use a cut off wheel gradually going around the cable in the direction "with" the spiral of the cable, so as to not fray it, working around and around gradually until it is cut clean. Then install a vacuum plug over the cable end to help keep it from fraying and give a finished look. Geez this is long, I hope it helps... Last thing/idea... if there's still no engagement you might have to either get a bigger pulley at the end of the handle pull tube, if possible to fit a bigger one, or craft a pulley of sorts underneath the car to provide more cable pull/leverage for engagement.

Sorry for length but hope these thoughts help. Can't wait to see what you come up with