69 charger. Starts, but when key "relaxed" to run position...dies. tried new switch. Plenty of fuel...ballast resist good, new ecu box? Thanks for any help!

There's 2 wires that run from the ign sw. to the ballast, one of them provides full 12V during cranking only, the other is the run only power wire, they a brown and dk. blue. You run power wire is the problem. find it and youre gloden. Could be switch....

Just went through the same problem on my car. Turned out to be the blue ignition wire coming from the switch to the harness. I could see a slight burn mark on the plastic connector. I cleaned it up and it was good to go. Same problem as you describe, it would start and then die when i released the key. Best of luck !!!

seperate the bulkhead & start there/check for fire/continuity to see if the blue wire ign1 (run) circuit OPEN is upstream back inside under the dash to the ign sw or downstream from bulkhead toward the ballast. check terminals/connections at wire ends. Unless there's been burnt strands from too much amperage flowing or broken strands from excessive flexing from not enough clamps or broken strands from normal flexing but wrong type of wire (not flexible enough type of wire for auto use (rare) or a dead short from broken insulation allowing a strand(s) to ground out, wires (metal strands inside) do not go bad simply from old age unless the insulation outside is REAL old & even then something has to affect the strands. grab your meter