Where is placed the thermostat for an audi A4 from 1998?

Water temperature is rising after I drive a few miles,then the light for checking the oil and water is on.Sometimes even the light for checking the engine too.What could it be ...the problem.I also have leaks from my radiator.Where is placed the thermostat for an audi A4 from 1998?Could be a sensor picked up a problem with the oil pressure and cooling system, get it to a reputable repair facility asap.Where is placed the thermostat for an audi A4 from 1998?Go to a RADIATOR SHOP not an auto repair and get it checked. NEVER replace any part till you know the problem.