In most cases there is no "wrong way" if you're riding on the sidewalk...pedestrians don't get ticketed for walking the "wrong way", and unless there is a law specifically requiring the cyclists to ride in the same direction of traffic I'm pretty sure that's not a requirement on the sidewalk.
In my town it's a crime for a person above a certain age (I think 12) to ride a bicycle on the sidewalk in a business or residential district (which covers most of town). Check your local laws.
That said, even if you broke a "no sidewalk" law, if the presentation is correct the motorist failed to stop at the stop sign, and also failed to yield to a human being in the crosswalk directly in front of his/her vehicle, so I'd place most of the blame on the motorist.
If it was legal for you to ride on the sidewalk, and you are not required by law to dismount at crossings or ride in the same direction as traffic on the roadway, of course even more of the blame would be on the motorist, because they should spend extra time looking for wrong way cyclists flying through the crosswalk if that is legal in your area.
A few details that could sway the amount of blame. How was the lighting at the time? If it was dark, was the bicycle equipped with the legally required reflectors and light(s)?