Does anyone have any experience using a dual band 2 meter/440 glass mount antenna on a window with those defroster lines in the glass? I'm concerned they may cause SWR issues running between the antenna and the coupler on the glass.
It will make a difference. First, what year is the car, and what make? If it's a fairly new vehicle, passivated glass will most likely have been used for the window glass. That alone will make the antenna much less efficient. The defroster lines affect performance even more.
If you're set on using such an antenna, try to mount it where there are no defroster lines--if it can be done, on a side glass. Other than that, I would recommend using another type mount altogether. Trunk lid mounts are better, but the very best way is to drill the hole and permanently mount the antenna!