There are bulkhead connections to get past the firewall. When I rebuilt my PA22/20 into my commute-mobile, I put in a stainless steel firewall and used pressure bulkhead plugs for everything to include the electrical. I had Electronic International engine instruments and used one of the available data points on a UGB16 as a firewall heat sensor. It worked well.
I would like to have
Aviation's Enterprises Mirror System for Fire Detection, but at $1K for the mirrors, nope. I'll do something like I did before.