temp. speed control

i have a 2000 blazer 4x4 and so far i've had to replace almost everything you guys talk about..... i have this one problem... i have 1-3 speeds on my climate control but not the 4th speed.... i've replace the relay, the actual switch and even rewired it but it still doesn't work...anybody have any idea what it could be? such a shame that a 23-28,000 dollar truck has such bad engineering.... any ideas will be well appreciated