This is the wiring diagram,
http://www.madracki.com/miata/images/wiring/92diag.pdf
If you go to page 3 c-11 It's the wiring for the interior lights.
This step isn't quite necessary but I replaced my door jamb switches because they were failing:
In order to get them to connect I had to add a new end to the wire :
New door switch on a new connection :
As for the wiring, if your exisiting map lights are working, your best bet is to simply add the wires for the lights you want to work, and put them on the terminals of the housing for the stock bulb. This will retain all use of the switch and simply just make what would be the bulb, your lights.
From left to right :
Constant on, door, off
From left to right:
Constant on, door, off
And if you were wonding, from what I gathered, this is your set up, but I'm not 100% sure :
Your red up top is the constant power feed, your white is the feed for the door switch and the black is your ground. I'm 98% sure that's right, but I'm not definite. Don't hesitate to use the wiring diagram above to help you out.