Nissan Repair Questions? Ask a Mechanic for Answers ASAP
It's most likely related to the blend door. The HVAC control head is probably getting and out of position signal from the blend door.
The only way to tell for sure is with a compatible scan tool to check the trouble code that the HVAC unit is indicating is present with the light.
You could also just replace the blend door actuator if a scan tool isn't available, but I don't think they are very easy on the 2007 frontier.
Do you plan to fix it yourself?
I would scan for trouble codes and see what the HVAC head is saying.
Please let me know if you need additional information.