Cruise Control not working on a 2003 Lincol Aviator

You could have a vacuum leak in the hose which will keep the cruise control from engaging.

The easiest solution is to take it to a mechanic and have them check for leaks. They can use a smoke machine and hook it up to the line to see where the hoses might be leaking.

The 2003 Aviator does not use a vacuum controlled cruise. So forget that idea, & look for a cable off from the throttle body. If the cable is still connected, then likely the issue is the shift lock solenoid. Sounds odd, but the cruise control module looks for ground on the brake switch, & the brake switch controls the shift interlock. A floating ground has caused many headaches, untill finally found. In fact, there was a service bulletin, or tsb, from Ford about this. Unplug the shift interlock solenoid, & see if the cruise works. Then u know the problem.

Either speed control cut off switch, or servo, other than that, servo cable is broken or disconnected