Check-out page two of this brochure, under "Comfort Indicator."

http://www.ctek.com/PDF/folder_USA_low.pdf

The appropriate LED blinks, sorta like the alarm LED on the mirror, so I don't think it would drain the battery (since the indicator is always "on" if you wire one in.) It's also discussed a bit in the "Battery Charger" thread of the past couple of days. If I added the indicator, I would probably hook it to the jumper terminals/bolts in the engine compartment and route it to be near the grill/hood opening in the front.