Compass in mirror not accurate

I have a 2004 Trailblazer with the electrochromatic mirror with compass built in. When I got it, the compass only would read "NE" or "NW". I doublechecked that the zone was correct and it was. I also reset it to calibrate itself again and now it gives some different readings occasionally but mainly sticks on two or three directions. Does anyone know if this is a problem in the mirror or in a sensor somewhere and what's the best way to figure out what's causing it. Thanks.

Mine does this also, especially in some areas where the power lines cross the road,.

my 02 loves the SW only direction for some reason even if I'm going NE.