Here's a simple schematic for a warning buzzer that will sound if you've left your lights on. I'm afraid I can't remember where I found it, but if you've got a few spare relays and are handy with a soldering iron you shouldn't have much trouble figuring it out.
Note that only later MGBs have door switches. See Joshua's comment below for more info.

Schematic for Door Open Lights On Warning Buzzer
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