I recently purchased a stock am/fm/cd/eq (off ebay for a $100) for my mother's 99 Montana that had an aftermarket radio in it and the controls worked right away as soon as I plugged it in. I think as long as the plug is the same in the back you will be ok. It only has the one main plug and the antenna plug, so just make sure they have the same configuration by looking at it. Only problem with a chevy radio is the backlight will be blue and not match the orange lighting in your 99 Montana.
As far as the HVAC controls, what are those?
Hope this was a little helpful.
Slim
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