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 know this is from 6 years ago, but here's what I did to fix my friend's N3DSXL with the same issue:

Firstly, I held: L trigger, R trigger, D-Pad down, and A.

I then turned it on (while still holding that button combo) to boot into safe mode, where the 3ds system can be repaired from a failed update, or to even just update the system.

After updating the system, this then brought me to a touch screen displaying "An error has occured" upon booting up the console. To fix this, take out the battery for a few minutes, then re-insert (discharging the battery is not necessary).

After doing these two things, the N3DSXL will have been fixed, and have booted to the home screen.

Hope this helps anyone in the future!

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.