well i wired my boost gauge to the headlight switch for a power source, and the headlight switch came out easy from the outside. I just pulled the switch slightly hard and it poppeed right out. What i recommend is just cutting around the headlight switch so you can fit it back through the same hole and just secure it on something inside the dash, so its still there and you dont need to mess with electronic stuff too much. Then just pop the boost gauge in the place where the headlight switch was and wire it up. hopefully the gauge fits in there.
2000 Saab 9-3 Viggen Sedan
-BSR Cat-Back and Intake