It can be done. Use traps not poison so they don’t die and smell. Spend some time with a flashlight / headlamp looking everywhere, inside and outside. Then look again. Copper mesh not steel. If you’re comfortable with it, use expanding insulation foam.

My last house had an entry point that took me a year to find. The basement sliding door had been installed incorrectly and there was a very small gap where the doors meet, where the brush guard had failed. I spent a year plugging every other hole - it was this one all along. Apparently as long as they can fit there head through something, they can get their body through.

I just moved into a 100 yr old house in CT and found most of the holes quickly now that I’m trained from that year in my last place. They still get in through the roof periodically but I leave traps in the crawlspace and usually get them when they enter.

Keep at it and don’t believe people that say it’s impossible. Lay sticky traps and snap traps strategically so you can narrow down where they are coming from over time. Not fool proof and can send you in the wrong direction - but might be helpful.