CEL on and ambient temp gauge blank when rear defrost turned on

This is a strange one. 2012 500c 58k owned since new. When you press the rear defrost button, the check engine light immediately turns on and the ambient temperature read out goes blank. When you turn off the rear defrost, the CEL eventually turns itself off and the temp gauge reads extremely high and then eventually drops back down to normal. This is reproducible every time you select the rear defrost.

Anyone have any ideas?

I've heard of clusters showing incorrectly high temp readings but not the other issues you speak of. The remedy for the gauge issue if I recall was a ecu reflash or replacement of the cluster.

The cluster is digitally mated to the body control module, airbag module, and ecu via VIN number, so when instrument cluster work has to be done, the cluster has to be sent out to Fiat with the vin number so it can be reflashed to match the previous cluster.