Why does the inside of my honda civic 89 smell like burning plastic/rubber?

[bewildered]

I was driving the other day and the cabin of my car started to wreak with the odor of burning plastic/rubber. I smelt outside to see if it was my brakes, but I didn't smell anything outside or under the hood. I had the heater going and the lights were on. It was also raining pretty hard if that has any significance.