Im trying to fill up the engine coolant on my sister c220d w205 model. However there are 2 reservoirs with the coolant label on them. If you are looking at the engine there is one of the left and it's black. There is also one on the right and you can see inside it but it's smaller than the one on the left, it's also empty.. I'm not sure which one to out the coolant into and also what the two different reservoirs are for. There is nothing written in the owners manual about the reservoir on the right. All help appreciated. Thank you.