This Act shall not apply to vehicles used by elective State officers, by executive heads of State agencies and departments, by presidents of colleges or universities placed under control of officers of this State, or by any employee of a State agency or department in the performance of investigative services exclusively when the executive head thereof has requested an exception in writing, and such exception has been approved in writing by the Department, on the basis that the identification would hamper the individual employee in the routine performance of his investigative duties. A record, open to public inspection, shall be kept by the Department of all such exceptions approved by it.
     This Act shall not apply to vehicles assigned to the use of the Illinois State Police and the Division of Law Enforcement of the Department of Natural Resources, and the executive heads thereof shall have within their discretion determination of the type of markings or identification, if any, to be affixed to vehicles assigned to said Department or Division nor shall this Act apply to vehicles assigned to the use of Secretary of State police officers.