Check to make sure you haven't shorted the antenna amplifier which resided in the back of my 2005 Outback. Also try disconnecting the new light you hooked up -- it is possible it (or an accidental short) is pulling down the voltage on that line. I would think that would burn out a fuse...but to burn out a fuse you need to draw the rated current for that fuse to burn out (e.g. 20A needs to be drawn to pop the amp with "20" on it); if the supply on that bus can't supply 20A to begin with (e.g. it can only supply 10-15A) then it'll just sit there shorted,, pulling the voltage low and not popping the fuse. If the bus can supply enough current to pop the fuse, then maybe your new light is 'open' with a low impedance -- if that's the case you should add a current limiting resistor in series with the new light to make sure it will only use as much current as you choose. Choose your resistor with R = 12V/I where I is the current (in amps) you want to limit the new light to. If you're just not sure if that's the cause, pull out a multimeter and check ther resistance between the 12V you used to connect to the V+ of your new light and GND anywhere on the frame. If the resistance is low (< 10 ohm?) I would be concerned as it COULD (but not necessarily) imply a short. If I read a low resistance, I'd personally start looking at the wiring i did to see where I screwed up and didn't wrap bare 12V wires well enough to keep them from touching the frame.
Sorry if that is a little scattered. Just problem-solving mode