I have an aftermarket cruise-control system which only somewhat works in a 2005 Ford Focus.
By this I mean that it controls the speed but is largely inaccurate (+/- at least 10mph). Going up/down hills causes the speed to slowly increase/decrease for a long while. So it works... somewhat, but, obviously something is broken/failing with the system.
What can I do to diagnose the problem?
It looks identical to this product. I do not know the exact model because it was installed prior to me purchasing the car.
Here are some pictures
I think this is the unit under the hood (I'm not sure though...).