If I see M3A1 or Snipers early I go 4 units into T2 Scout Car with Upgun. Usually that can bag the car. If it comes later it's harder to deal with but mortars and mortar tracks can help.
All in all, I think snipers in cars need to be nerfed.
Seriously. Either relic makes it impossible for snipers to enter the truck, or Sniper miss 2/3 shots when in it.
It's not even skilled playing, just sly playing.