the vac hose from your intake manifold.
it runs down to the battery area, and depending on year (I think) it will split into 2. one goes down to the bumper and attaches to a vac cannister behind the bumper itself.
after breaking my cannister I noticed the cruise would not engage properly, it would engage but not very well. it wouldn't pick up when I pressed accell, and up hills it would slow down quite a bit.
I have actually removed the cannister from the equation, where it splits to 2 hoses I took the one going to the vac can off and instead of the y splitter I just pluged one into the hose going to the manifold. so far it works just fine, I will track down another vac can and put it back in on the off chance it was serving a purpose tho.
so as previously said, check all the lines to make sure you don't have a leak. as silly as it sounds try pressing harder on the button too, my Cruise On button has to be puched hard and a few times for it to come on.