Voltage from ingnition to starter relay?

In the start position the ignition should see a full 12 volts, 6-7 volts to the ignition in the run position is normal. That means the ballast resistor is good and doing its job. What is referred to as the starter relay switches the ballast resistor in and out of the coil feed circuit. You have a 6V coil. Chrysler did this to improve cold starting by temporarily running a 6V ignition on 12 volts (which drops to 8 or 9 when cranking).

The only components that can cause the starter to engage are the ignition switch and the seatbelt interlock. Sounds like the switch is working, so that only leaves the interlock system. If you really want to keep the interlock you will have to find a good control box, sounds like yours is bad. If I knew where my old Charger was I could get you one. Never used the interlock on that one since it was disabled before I drove it off the dealership lot. The dealership couldn't disconnect them but I was told which plug to unplug when I bought the car. According to the service manager, the seat sensors had to be disconnected to disable the system without causing problems. The problem you are experiencing was one of the reasons the interlock system was a one year deal. The only good thing I can say about the system is that it got my girlfriend to wear her seatbelt. Never told her it was disconnected.