Incorrect heading indicated on mirror compass?

During our last outing while driving about 150 miles, we noticed that the compass in the rear view mirror was showing some obviously incorrect headings.  Sometimes when we knew we were headed almost due north, the compass showed that we were headed south-west.
I talked to Kermit about this.  He thinks the errors are caused by the compass sensor being placed too close to the radiator.  When the radiator gets hot, the heat causes errors in compass heading.  Kermit said I could reposition the sensor that is located under the front bumper (driver side), but I can find no sensor anywhere.
Has anyone had any compass error like this?  Can you tell me where the sensor is located on your PC?
--Bruce

Don't know about the sensor but do know that longitude and latitude effects the accuracy of the compass. Somewhere in the literature you we're provided  when you bought the PC is a brochure on the mirror /  compass that explains how to reset it for the area where you are traveling. Sorry can't be more specific.

I've not had good results with a mirror compass of any manufacture because it relies on magnetic lines of flux, which change depending on where you are. Consequently, I installed a GPS on the dash using a north up setting and can see my heading and how that relates to my position.

I too use the GPS map north up. I can tell if I am going in the right direction. I cannot stand looking at the 3D maps spinning around as I turn.