I know about the order necessary for placing a GPS mobile tracker on a vehicle....but...would it be necessary for placement on a county owned/maintained vehicle issued to a county employee for county business if say the department head consented to the placement???

Generally no. Best practice would be to inform all county employees that county owned vehicles may be equipped with such a device. If the county has a policy restricting personal use of county vehicles, that should also weigh in favor of the ability to place the device on county vehicles without any order, since there is an even greater reduction in any possible expectation of "privacy".