I don't think the REST led lights up with key on or engine running / pressed-held for 5 or so seconds to get to the data screen. With the key off the REST function should engage, if it won't, perhaps the SW is failing. The HVAC needs to be on ( fan running on low ) for the test to activate.

Past that, a generic OBD 2 scan tool will get you coolant temp.

The control head might have a problem, I'm pretty sure it is the same as similar era C and SL but needs the coding changed. Also pretty sure the coding can be changed using the buttons and not a scan tool.