Battery cables

Hi! New member, so bear with me if this has been addressed. I have a 97 Camry 2.2L 4cylinder. The cable from the battery to the starter was corroded and broke when I tried to clean it. I have replaced the cable and the clamp, the battery shows 12.5V, but when the pigtail (smal wir) is plugged in the voltage drops to 7.4. sounds like a circuit open. How do I find the problem? Of course, car won't start. Thank you for any help.