Gas fumes in the cabin of my car

I have a 1996 Volvo 960 station wagon which I love dearly. It’s been a wonderful car, however something new has occurred that has me quite concerned. Whenever I turn on the heater, I smell gas fumes. This doesn’t occur when the A/C is working or when I turn the temperature dial beyond the numbers to the red mark. I understand this is a different heat source than the regular heater. When I took my car in to the dealer for its 150,000 miles checkup I told them about this problem. They experienced it, but couldn’t find the source of the challenge. They want me to bring it in again and keep it overnight so they have a lot of time to decipher the source of the problem. Could you give me some of your insight since you’ve dealt with so many car problems?

Have someone check for fuel injector O-rings that may be leaking. It takes very little gas on the hot engine to create that odor.