You know those blinking LED's that's on the box of the Logitech optical mouse? If you dont have one already, you should buy one today. It's only 25 bucks at Best Buy.
Anyways, I ripped out the LED and put it in my car so the blinking red light looks like an alarm during night.
The led is attached to 2 double AA batteries in a holder by a 3" wire. I think it'll keep blinking for a long time.
During the day, I keep it in my secret comparntment (below the stereo and above the ashtray) and at night, I just pull out the LED.

hmm, maybe this sounds stupid, but I like it anyway because it looks cool at night. ha ha.

 

Well, for like $5 you can buy a device (found at most auto stores) that does exactly that...uses two AAA batteries to power two small flashing lights. It looks cheesy, but like you said at night you can't really tell.