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Silverado 2500hd no cruise control or elevated idle

i bought my second used 2007 chevy silverado 2500hd classic LT 6.6l diesel and the cruise control and elevated idle wont work. have done tons of reserch. And have found nothing useful to my situation. have found everything from a simple fuse to a module which is expensive. i have checked that all my brake lights work, have checked all fuses and even bought a new colum switch for the cruise control. that didnt fix it. i am thinking the brake switch or possibly something with the trailer whiring might be shorted or installed incorrectly.the reason i think it has something to do with the brakes is it is a common denominator between the cruise control and the elevated idle. if elevated idle is on and you touch the barke petal at all it shuts off and idles normal on a working system. which i know from my previous identical truck. the brakes also shut off the cruise if touched at all. i have had a hell of a time trying to figure this out. any help would be greatly appreciated. thanks tucker

It could be the switch that tells the computer that you’re pressing the brake, but it could also be the cruise control mechanism, which basically controls another throttle cable, and hence the engine speed. You might be able to test the brake switch idea if you turn the key to ON, but with the engine OFF and try to shift out of park. If it shifts, it’s definitely the switch, although I’m not totally sure that it uses the same switch for both the shift interlock and cruise control. Those also tend to break the other way, i.e. the switch won’t register that the pedal has been pressed, so I find it a bit unlikely that it’s the switch.