What is the test for determining whether a party was at fault in a motor vehicle accident?

Appellant requested that the trial court instruct the jury that, if it found that Ruhof was at fault and its fault ... was a substantial factor in causing [respondent's] harm, it should assign percentages of responsibility to both appellant and Ruhof. [U]nless there is substantial evidence that an individual is at fault [and its fault was a substantial factor in causing harm], there can be no apportionment of damages to that individual. (Wilson v. Ritto (2003) 105 Cal.App.4th 361, 367, 129 Cal.Rptr.2d 336.) The burden is on the defendant to prove that a nonparty tortfeasor was at fault. (Id ., at p. 369, 129 Cal.Rptr.2d 336.)

Substantial evidence necessary to support a jury instruction is evidence

[182 Cal.Rptr.3d 711]