In an embodiment, a printing method for forming a conductive line on a plastic substrate for use in an automobile, comprises one or both of inkjet printing and screen printing the conductive line on the plastic substrate while the plastic substrate is fixed in a fixture of an automatic printing machine; the conductive mixture comprises a conductive paste; a solvent; and an adhesion agent; and wherein the conductive line is a busbar, a grid line, or an antenna line in the automobile.