Have you checked the "interior illumination" fuse yet?
I'd say you might be shorting somewhere in that new boost guage dude. Remove the wiring, then change the fuse, and leave it like that until you can check it out.
Best to use a multimeter when doing ANYTHING electrical, in my opinion. Strap the red wire on, with everything off, and then do a continuity test. If you use the multimeter on the red wire at the boost-guage end, and then use the black multimeter wire and touch part of the body, it shouldn't beep. Then put the red multimeter wire on the black wire to the boost guage, and the black multimeter wire to the body of the car, and it SHOULD beep... that tells you that you are testing on a good earth spot.