What would cause it to work one day and not the next(cruise control) and where is the speed sensor located

You do not have a cruze module like the old days. The throttle body is an electric motor and is controlled by the ECU

It's probably a problem with cruise control module , located under the hood in engine compartment . Or a bad connection somewhere . Do you know how to do electrical testing ? Is the air bag light lit on the instrument cluster ?

Could have DTC'S - diagnostic trouble code's stored .
DTC P1554
The powertrain control module (PCM) inhibits cruise control operation when certain conditions are not appropriate for cruise control operation. The PCM removes the ground from the cruise control inhibit signal circuit in order to disengage cruise control operation.
DTC P1585
Output driver modules (ODMs) are used by the powertrain control module (PCM) in order to turn on many of the current driven devices that are needed to control various engine and transaxle functions. DTC P1558 set indicates that an improper voltage level has been detected on the signal circuit which controls the cruise control inhibit circuit.
Action Taken When the DTC Sets
• The PCM will not illuminate the malfunction indicator lamp (MIL).
• The PCM will store the conditions present when the DTC set as Failure Records data only.

Cruise control is a speed control system that maintains a desired vehicle speed under normal driving conditions. However, steep grades may cause variations in the selected speeds. The electronic cruise control system has the capability to CRUISE, COAST, RESUME SPEED, ACCELERATE, and TAP-UP or TAP-DOWN.
The main parts of the cruise control system are:
• The cruise control module assembly
• The function control switches
• The cruise control release switch
• The clutch pedal position switch (CPP), if equipped
• The stop lamp switch assembly
• The vehicle speed sensor
The cruise control system uses the cruise control module assembly to maintain the desired vehicle cruise speed and operation. The cruise control module assembly has an electronic controller and an electric stepper motor to vary the throttle in each different cruise control mode. The electric controller monitors vehicle speed and operates the electric stepper motor. The stepper motor operates in response to the electronic controller, to maintain the desired cruise speed. The electric stepper motor moves a strap that is attached to the cruise control cable which moves the throttle linkage. The cruise control module assembly contains a low speed limit which will prevent system engagement below a minimum speed of 40 km/h (25 mph). The module assembly is not serviceable.

cruise controls are dependent on the following sensors all working together
brake sensor
clutch sensor
auto--vehicle speed sensor
rpm's sensor
fuse for the cruise control
cruise control switch either or all not working will stop the cruise control from engaging
road speed at which you can engage the cruise control has to be above a reset speed before it will cut in and work

Year make model

The one part that can affect both your cruise control and your speedometer is the vehicle speed sensor. The MOST LIKELY cause is failure of the speed sensor itself (very common). However, I would not run out and spend money on a speed sensor before verifying that the electrical circuits leading to and from the speed sensor are working correctly. The problem might be cause by a broken wire or a loose connector (also relatively common).

The sensor is located at the left-hand side of the rear of the transmission housing.

An emission control sensor cannot cause the cruise control from working. They are two separate systems.

However, if the engine rides rough or bucks, then it will be difficult for the cruise control to regulate speed on the highway.

cruise control operation is reliant on the following
road speed sensor ( speedo or VSS sensor )
engine rpms tacho/ ignition coil/module problem
clutch pedal sensor (VSS in automatic )
gear selection position sensor ( not properly adjusted causing flickering of the selector light)
brake pedal sensor
bad ground wire connections
r all need to work together or the CC will stop working
nothing to do with vents and all that you mentioned.
wire connections and harness connections for the transmission are suspect
CC will not operate under certain engine rpms and road speed

Check the speed sensor at the tranny or near left front tire. Use a manual or find the location on the net. This sensor usually causes problems with the cruise control.

Check to see if you have any relays. Also make sure vaccuum hose is still connected.

i would take a look at the speed sensor because the cruise reads its signal off of this sensor