When the harm is severable, each party will be at fault according to his or her degree of harm caused. However, when it is difficult to parse the harm, the court will find either party liable for the whole harm; at that point, each Defendant will attempt to prove liability for only X amount of harm (equivalent to the amount of harm caused by that Defendant).
This is the stuff that trials are made of. Fortunately, it sounds like a small claims trial.
Driver two will argue that driver one was backing unsafely, and struck his/her parked vehicle. Driver one will argue that driver two pulled in without paying proper attention. Both drivers will be complaining the other driver was negligent for failure to maintain a proper lookout and exercise due care. The trier of fact (judge, jury, or in the case of Safety Responsibility proceedings, hearing examiner) will need to decide who to believe, and then apportion a percentage of causal liability for the accident. It needs to be at least 51/49 for someone to "win" and damages will be set accordingly.
In short, if both drivers had insurance, and neither driver was injured, this is precisely why you pay premiums. Let the insurance companies handle it.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.
Under Wisconsin law, we have what is called comparative negligence, which means that the potential negligence of both parties will be evaluated to determine what percentage of 100% might be attributable to each. The issues of Lookout and Management and Control will be the legal issues involved. Under Wisconsin law, each driver essentially has a duty to maintain lookout all around them to see any other vehicles and determine if another vehicle might potentially be involved in a vehicle collision with them. If a driver maintains proper lookout and observes another vehicle which might potentially be involved in a collision with them, they then have a duty to manage and control their vehicle in an attempt to avoid a collision, including such things as sounding their horn, applying their brakes, swerving, etc. The less they do, the more negligent they might be, and the more they do, the less negligent they might be. I suspect that these are the two legal principles in play in your situation, which will probably make both drivers somewhat at fault and negligent. If both drivers are negligent, then the fight shifts to determining or agreeing on the percentage of fault of each driver, adding up to 100%,
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