Your problem is that you've been lazy! You've done one side of the car with all the decals the right way round and then you've just copy pasted and flipped the whole side of the car (including decals), hence all the decals on the left of the car are backwards!
You need to; delete the left side copy, hide the decal layer(s) and then either copy the right side (without decals) paste and flip onto the left hand side or just draw it again, then show the layers on the right hand side but as you do each one you need to "duplicate" the layer onto the left side and then "transform" to suit the other side!!!!
Hope this helps!