I used to smell mine periodically and same issue....no wet carpet and no exterior leaks. Mine happened to leaking at the grommett leading into the cabin and running under the carpet. Go figure. Was hard to find. When I found and replaced that hose, my core finally decided to take a dump a few weeks later. Hence why it is sitting in a box collecting dust right now.
But with these posts, I have since decided to go ahead and run parallel fans with a switchable circuit and contact Gary for a PWR rad. Better safe than sorry.

 

Speaking of parallel fans, I had to install slim fans when my Koyo went in, and I wired them in parallel. They don't turn on with the AC now, but I don't see that as a problem, since the ecu will activate them both once the set temp is reached.

C