Ah, that is hopefully easier.

The main +12V red lead to the ignition runs via the battery + terminal via a "fuselink" wire. This wire should be the only one that is black (and quite thin) in colour. The 1st 4-6 inches are the fuselink and are designed to break if overloaded. However, they do degrade. I would check the lead where it goes to the battery and see what its condition is. Chances are it has broken where the connector is.

From what you wrote, I assumed you were putting the supply @ the ignition, i.e. the red lead had +12V. Should have asked previously really.