Does backround count affect either of the sides TNs in an opposed test for a spirit power such as Influence? I tend to think backround count would affect the spirit since the spirit is actively manipulating mana but not the target since the target is just resisting not actively manipulating mana. Thoughts anyone?

A fast scan produces no evidence either way, but I agree with your ruling. The spirit gets TN penalties (just like a spellcaster), and the target doesn't (just like a spellcaster's victim).