light smoke coming from gear selector area

there are a few things to check before worrying about the auto box which is normally a very robust unit. 1stly the colour of the auto transmission fluid which should be red 2ndly the level of the fluid, both can be checked on the dipstick, dark brown fluid would indicate that at the very least the fluid has not been changed for some time. Next get underneath and check for leaks or if you park in a particular spot is there a patch under your car.
Check for leaks with the auto in park and the engine running, as it may well be that you have an exhaust leak and this is finding its way up in to the car. Proceed by way of elimination, a serious fluid leak could give rise to burning transmission fluid if it has become very low. Does the auto box shift up and down through the gears reasonably smoothly? If the fluid is low keep it topped up and if all the above are negative and you are still worried get it checked by your local Merc specialist.