Headlights malfunction when heater on high

After truck warms up inside, under the dash area gets way too warm, causing the headlights to go off and on, maybe from the light switch getting too hot. I turn the heat selector over to cold and it cools off the dash, then the lights work again. To prevent this problem, I can't turn the heat up past around 65 degrees coming out of the duct, barely putting out any heat at all. I can't have a stereo in the dash because it gets too hot and fries the unit.

Since I live in Alaska, every winter eventually hits -40, 50, or even 70 below zero, and I really need the heater to put out some heat so I don't freeze! Why do you think the under dash area is getting so hot? Should I insulate the ductwork? Thanks, Tim