From the 06 owners manual:
If the vehicle is driven and vehicle speed exceeds 19
mph (31 km) before the manifold heater post-heat
(after start) cycle is complete, the manifold heaters will
shut off.
https://www.mopar.com/dodge/en-us/care/owners-manual.html?openGarage=true
If the grid heater comes on randomly while driving above 19 mph and after the heat cycle is completed, you may have something else going. Are any codes being thrown?
If the vehicle is driven and vehicle speed exceeds 19
mph (31 km) before the manifold heater post-heat
(after start) cycle is complete, the manifold heaters will
shut off.
https://www.mopar.com/dodge/en-us/care/owners-manual.html?openGarage=true
If the grid heater comes on randomly while driving above 19 mph and after the heat cycle is completed, you may have something else going. Are any codes being thrown?