[flatford8]

You didn’t mention if it doesn’t restart when the engine is hot, or at ambient temp. Start by having the battery tested. Be sure you have good clean cables (rated for 6V )and connections. Same with the grounds like TJ said. Then test the starter draw both hot and cold like Tubman suggested.The 6V systems will work fine on these cars as long as everything is in good shape. I don’t think there is a need to try to “splice in” a 12V to start your car………Mark