2 ways, Brother.
They sell a tool that injects propane into the tunnel, then sets it off with a pizio ignitor (we used to do this with just a propane tank and a burning wad of toilet paper... Not recommended  )
Method 2, run a hose from a gas engine (make sure it can handle the exhaust heat), and stuff it into an active run. Plug air leaks as you find them. 10 or 20 minutes, lights out.