Hi Scotty, I have a 2008 Toyota Matrix with a standard transmission with 125k miles on the clock. My cruise control was working perfectly as designed when I bought the car but after three months of ownership it will only turn on (the cruise light will turn on at the dash) but will not engage or hold a set speed. I have replaced the steering wheel cruise control switch with a used one buy same condition persists. What could be cause for why my cruise control won't set?
Thank you so much!!!
I would check the fuse for the cruise control and the speed sensors.
Thank you so much, I will check that out.
There will be cruise control cutoff switches on the clutch and brake, though the brake light switch may be used for the latter rather than a separate switch. Check that those are operational.
Thank you for your advice, I imagine that I would probably probe the connector to the brake switch with a voltage meter with key in on position then depress brake pedal and look for voltage change to test operation of the brake switch?
It would be preferable not to to probe live circuits in a modern car unless you're sure about what you'redoing. What you could do is leave the car turned off, remove the switch's connector, and check operation with an ohm meter. Same for a clutch cutoff switch if present.
If the cruise control cutoff is being triggered by the brake light switch another symptom would be the brake lights staying on while driving.
Thank you, I will definitely try that, I might even take drivers seat out to allow more room to work in that cramped space 
After much delay, I was able to install the new clutch switch and it fixed the cruise control. The wierd thing is that the clutch switch came out of the car had continuity like the new switch. I'm guessing that the old switch was not mounted close enoughto the clutch pedal contact point and thus not closing the loop to the circuit
