In the beginning of the movie a pair of travelers are walking through woody terrain covered in deep snow, well dressed for cold winter. They reach some sort of bunker and go inside. There it turns out they are a man and a woman, and that the red-hued interior is unbearably warm. They are forced to remove all their clothing except for the underwear. The woman asks why he haven't warned her about the heat, and the man replied that she wouldn't have believed him. They are there to break a safe. The safe has a keypad with matrix with a lot of keys, maybe even 10x10=100 keys. One of them presses some of the keys in appropriate sequence and appropriate pattern, and the pressed keys are lit up.
This happens in the first 10 minutes, or so. It's probably a spy thriller from the late seventies.
EDIT: user Anderson reminded me of one more fact. After removing almost all their clothes they poured a liquid (could be a chilled champagne) on their bodies to better manage the heat. Anderson also mentions some special fuel pellets causing all the heat, and it could be true, but I have no memory of that detail.