1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
Yes,It's a two person job, one pourer one inspector. It's not a bad idea to start with a little at the top of the windshield and work down to the grilles.There are sometimes other leaks.Basical
Just pour some water via each fresh air grill, and within a few seconds it should drip on the floor. If it doesn't than the drain is / are blocked and you'll need to unclog them, per rowdie's post / RD's picture.