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I think it's the gmmp engine loom hahahaha

Do u have starter relay
I never run one if its manual unless car has immobiliser
Which I'm sure most cobras have
So could be something to do with immobiliser or how u wired the starter

If your lights are dim just run a temp earth direct to batt neg to see if it fixes it

I have found sometimes if battery is in boot there is no real good spots for an earth to chassis and I've once hooked an earth to the frame under roll cage where seat belts bolt to
And because it goes thru fibreglass I had a bad earth

Tested it by using two jumper leads as earth from batt neg to motor and fixed it so then I found fault and cleaned up bad earth

But to crank engine over it must be bad earth which makes power trigger your start relay

I wire my start relays different to some people so less chance of this happening