[Tommy]

Another test for the suspect cable is to run a jumper cable from that battery post to the corresponding place where the cable goes (e.g., from negative post to engine ground). If you then get lights, you know the cable was the culprit.

Here's a second thought that probably doesn't apply to your Cobra. Modern cars with computers require a minimum battery voltage to even attempt to start the engine. It's conceivable that a weakish battery might have enough voltage to satisfy the computer in one car, but not enough to satisfy the other. Thus, you'd get a start in one car and a no start in the other. But you should get lights and accessories in both cars in this case.