I've found myself turning on the dome light just to see my ashtray, or to access something on the left panel (mirrors/traction control) so I've been thinking..... what's the easiest way to add a little lighting in those spots?

 

You could use a single LED with a its own switch or wired into another switch you may have. I would use a larger than needed resistor to drop the power going to the LED, this would act as a dimmer as the light needed is not that much.

 

single led's are definetly the way to go.