Mercury is used by automakers in switches for convenience lighting (i.e. trunk lights), anti-lock brakes, and active ride control systems (i.e., adjust suspension when going around corners), and high intensity discharge headlamps. Consult this link for more detailed information on which vehicle manufacturers have used what mercury switches has been put together by the Clean Air Foundation.
Summary of Federal Requirements
Mercury is regulated from many different perspectives; as waste, as a water contaminant, as a human health exposure concern.
Waste material that exhibits the characteristic of toxicity for mercury and discarded chemical commercial products containing mercury must be handled as a hazardous waste (40 CFR 261.33).
BUT, under the Federal Universal Waste Rule adopted in 1995 a limited set of items were identified as being able to be disposed of as Universal waste instead of hazardous waste. This Universal Waste classification includes mercury thermostats and mercury vapor lamps.
Summary of State Requirements
Each state must establish minimum water quality standards for priority pollutants, one of which is mercury. The States are also taking the lead on mercury reduction by working with scrap metal collectors and other entities working with discarded vehicles to prevent the leaking of mercury from the vehicle to the environmental. For further information on how the states are managing automotive switch removal, see the End of Life Vehicle Solutions Corporation (ELVS) which was created by the automotive industry to promote the industry's environmental efforts in recyclability, education and outreach, and the proper management of substances of concern and provides state-specific information.