No Cruisin' in my '85 Chevy Van

[Radarguy]

I have owned this 85 Chevy Van all its life and when I take it on trips I like to use the cruise control. The last few years the cruise engages and works except that it seems to hunt or vary the speed up and down a couple of miles an hour. I replaced the servo figuring that it was just old and leaking vacuum. Well the new one behaved exactly the same. So what else is there? The electronics module? OK?!? I can see where you can buy a replacement but my only problem is; where the !#$? is it? I have looked everywhere under the hood and under the dash with no luck. Manuals don't even include the cruise system. Does anyone have a solution to the hunting issue or at least know the location of the module or any other suggestion?