CO mystery - any ideas?

Our Cessna R182 is getting CO in the cockpit. We've now had three different
shops look at it multiple times, all to no (complete) effect. They've done
things like resealing the firewall, the gear-wells, the exhaust, and the
like. But still we get CO.

We've swapped CO detectors around between planes, so the detectors' results
are trusted.

One interesting oddity: turning cabin heat on does seem to get rid of the
CO.

We've noticed no difference made between type of flight. I personally have
had readings both while XC and while maneuvering (practicing commercial
maneuvers), and at various altitudes. Others have reported the same.

At this point, we're at something of a loss. The next thing we're trying is
to extend the exhaust pipe. The presumption is that this would get the CO
into the slipstream and away. But that "presumption" is really more of a
"guess" or even a "hope".

Anyone have any suggestions or ideas or experiences that might help us?

Thanks...

Andrew