What we do and have always done is call dealers check for stock then ask if they will kindly make it available to an MB customer, we have done this for most vehicles over the last 20 years and 1 in 5 say yes including current car that we got last November. Be prepared to travel and take any colour but that said it prevents waiting. It is currently more difficult but certainly not impossible. Decision is usually made at branch level so often large groups will give a different answer at different branches.
I am the carer / driver for my wife who cannot drive due to disability