just been looking at this on my 156, not sure what your dash layout is but I found this link on how to get to the heater control panel on mine - same buld failed as the author of this had.
Fun starts @ 3000!
Trust Alfa to use a different bulb here than the restof the dash panel bulbs, so after dismantling I couldn't replace it anyway... grrr!!!!
Am considering LED's but they don't tend to throw enough light all around to work well, unless anyone knows better I'll try source the right bulbs and replace all 3 next time I feel inclined to strip it out again.
Dave