It makes no sense to cover the grill.
From an engineering perspective, application is designed and tested at minus 40C/F
Cabin, coolant, engine oil, transmission oil and fuel temperatures are eletronically managed and include back up heaters to bring them to ( and keep at) operating or set up temps.
2014 X5 30d M
2012 Grand Cherokee (sold!)
2003 MB SLK stick (garage queen)