Don't assume that it's not a bad ground. You need to check the ground strap from the battery to the body in the engine compartment and the big ground to the block which is in the middle of the car left to right and close to the front of the engine - you might have to take the splash guard off to access it - it is probably completely corroded. There is also a ground from the alternator to the block which might need refreshing. Now, if you don't buy this line of inquiry you can do a quick test - use a booster cable to ground the starter to the battery negative, then start the car with your key in the ignition. This is a definitive test for starter function. I know this because I went throught the trouble of replacing my starter ( at about 10 deg F) only to find that wiring was the culprit.

Ron
87 745 260 000 km
near Brantford, ON, Can