I didn’t want to name drop you again Secrop - but I suspected you might be along with a solution 
OP - just to be clear how Secrop’s method works.
As well as the car - you need to add an empty axes object to your scene (this will represent the radar gun) and select it in the object drop down of the texture coordinate node.
Now your heat map will point in the direction of the empty axes (radar gun) - not the camera - and you can therefore position the radar gun anywhere you like in relation to the camera and car.