Hide it in an empty windshield fluid reservoir, fuse box, taped up under thebattery, just don't pit it anywhere that gets hot. Seriously how often do they look under the hood?
I had an old honda that was falling apart. There were panels popping out everywhere that made good spots or I could put it behind the head unit. Also, some of the older cars have different keys for the trunk. My friend got pulled over and didn't have the trunk key because he just got the impala, the cops were pissed but they never called the dogs and he didn't have anything.