If you believe a car is abandoned: Speak with neighbors; they might know who it belongs to. If no one knows who owns the vehicle: report it to 911. Be prepared to give them the following information: location, make, model, color, license plate number (is the tag up to date?), city sticker (if it has one, is it current?)
Most important: is the vehicle in dangerous condition (a broken window or pieces falling off) and is it drivable (flat tire)?
The link for locating a vehicle is https://webapps1.cityofchicago.org/vehicleSearch/