Do people cross the border where there is no barrier or tunnel? Of course they do. Put up a wall and fewer cross. Easy fix.
Then, go onto the next issue with the tunnels. Blow them up. Put armed guards at the exit.
As for Michigan, I highly doubt the validity of the claim; however, increase the presence there.
Tunnels trump walls, so it's a waste to build them when your second move is to figure out ways that stop people from getting by them. We already have structures in place and they aren't working, why more of the same? address the reasons why they aren't working, and you'll find that new ones aren't necessary. New walls in 2019 is a complete waste of money.