My 2015 has one under the dash as Paulxj has mentioned, my earlier plus 4 had one in the glove box and I bought an armrest/cubby box from one of the dealers which had a 12v socket in, I just had to wire it up to the one in the glove box


2017 Mercedes Benz SLC 200 AMG.
2019 Mercedes Benz GLA 200 AMG
Now Ex-Mogger!