lack of coil signal

What would cause a lack of signal to the last two posts on the coil (top and bottom of the coil unit, closest to the firewall?) I own a 2003 Ford Mustang V6. The engine began to run sluggishly, as if not all cylinders were firing. After determining that cylinders 3 & 4 were not getting spark, I replaced the coil module, but the problem persists. The plug wires are fine and the other cylinders seem to be fine. The crankshaft position sensor seems undamaged. What would cause a lack of signal to the last two posts on the coil (top and bottom of the coil unit, closest to the firewall?)