Back 30> years ago we used to keep our car/truck cab warm with a 120 volt heater, These heaters would come with hardware to fasten them under the dash.
We are looking for one now for a special emergency response vehicle.
The remote start will not function for this application.
30 years later, they still sell them.
https://www.amazon.com/Zerostart-2600900-Interior-Electric-Portable/dp/B000NM73ZA/
https://www.amazon.com/Caframo-Heater-Profile-Powerful-Heater/dp/B004473UK2/
The 1st link is the one we need,
Thank you Very much 
We had them in our cars when I was growing up, but I never noticed much effect. But, maybe they’ll work in your situation.