Where to find wifi card on new 3ds xl?

It might not be the wifi card. If you have damaged ribbon cables underneath the left shoulder button, you may need repairs. I recently helped a friend with the same issue, and by messing around underneath the left button, I got the bottom screen to turn on. The top screen....I don't know. But if you get the bottom screen to work, I suggest not trying to do the same thing to get the top screen to work... *sweatdrop* (I ended up breaking it more after trying to get the top screen to work. I suggest being extremely careful. X'D)

I had this problem too. Even after taking the wifi chip or card off and back in, the problem would could back. Eventually I realized the wifi chip would get unseated since the foam that's supposed to hold it down was flattened, so I cut some more of similar foam down and put it inside. The issue has mostly gone away now, it only comes back every few weeks and I just have to press near the joystick where the wifi chip is located.