Occasionally Nigel’s overhead map lights will spontaneously illuminate for 10 seconds or so, and then go back to sleep. Is this just normal quirkiness for our beloved 27 year-old car? We did recently replace the rearview mirror with a new OEM one.

 

Mine has not done this but it does appear that a door switch is faulty and the lights think a door has opened and closed?

 

The most likely culprit is the door switch. Otherwise, you'll need to trace the electrical diagram (and the physical wires) to find the problem. Be patient. But as soon as it's fixed, let the community know.