1998 Chevy Blazer Odometer and gear selection illumination

I've owned my Blazer for over 5 years and never had a serious problem with it, but my odometer and gear selection have never illuminated unless it was extremely cold outside(i live in iowa). The guy I bought it from said it would come on once in a while, but that never happened. He mentioned that a mechanic had told him that they would have to tear the dash apart to fix the problem and it would be costly. It really never bothered me that much but I was curious as to what you might think it could be and would it be expensive? Like i said it never illiminates unless it is really frigid outside, like it has been this year. I thought maybe there could be some kind of electrical board heating up and the cold weather keeping it cooled down so it keeps a connection. It stays illuminated as long as it stays in single digits. Above 30, forget it. Thanks for any suggestions-Shaun