Heat is the Last Resort
Applying heat with a propane torch can expand a nut (or threaded opening) and loosen it. If you've tried everything and still can't free a bolt or nut, try heat. But first make sure the fastener is a good distance away from hoses, belts, gas tanks or anything else that might burn up your car or kill you in an explosion.
If the area is soaked with penetrant, spray it with nonflammable brake cleaner to remove it before you apply heat. Keep a fire extinguisher nearby. Once you've established that it's safe, aim the flame at the bolt head or nut, not the surrounding metal. Heat for about 15 seconds, but don't get it cherry red. Then spray the bolt head with water to cool it quickly. Continue spraying until it no longer steams. The expansion/contraction cracks the rust, so add more rust penetrant, let it soak and then add vibration by tapping it with a hammer or other tool. Then try to loosen it.