On my E430 (and most of the plastic tank M-B radiators) there is a red plastic "cap" for the drain valve. It has a screwdriver slot in it. Just slip a length of hose on the nipple, open the radiator cap, and open the drain valve. It is accessible from underneath after pulling off the forward section of the plastic undertray. To remove the undertray, drop the rear section (4 bolts) and then drop the front section (8 bolts - three in each wheel well and 2 at the front). It really is qiute easy.