Re: Mechanics question: cruise controls stops when headlights on?
Yep, many ford cruise controls use the same switch that controls the brake lights and cruise control; problem with that switch could do it. A broken brake light bulb also may "tell" the switch that the brakes are always on.
Another oddball is the switch is sometimes on the brake pedal, and bumps/potholes can jiggle the pedal, causing the switch to think the pedals been pushed; having the lights on might be increasing/reducing some electrical load in the switch that makes it more sensitive to this. Even a slight 'sag' in the pedal thats ok with the headlights off but not ok with them on might do it.
Note that these switches usually dont look for voltage to tell them the pedals been pressed or the brake lights have come on, they usually look for the appearance of a ground signal. So anything thats cut, broken or fused in the headlight circuit connected to the rear lights might be creating a "false ground".
Ugly, hopefully replacing that switch blindly does the trick, providing all the rear bulbs are working.
Be fearful when others are greedy, and greedy when others are fearful. Just another form of "buy low, sell high" for those who have trouble with things. This rule is not universal. Do not buy a 1973 Pinto because everyone else is afraid of it.