If the cigarette lighter or horn doesn't work, the cruise control won't work either - they share the same fuse.
Check to see if there is vacuum at the speed control servo that works the throttle cable that connects to the throttle.
Speed control servo installation – located in LH door pillar under fender
Check to see if the throttle linkage arm on the throttle body has 2 cables connected to it; one of them is for the cruise control.
Dump the codes to see if you get a code 27 or 29 - both of these are related to the VSS sensor that operates the cruise control and has some effects on the idle speed.
Dump codes sticky
Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes.
http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/
The diagram says 87-90 Mustang, but the 91-93 Mustang will be similar. The wire colors will be different, but the rest should be the same.
Speed control amplifier – located on driver’s side under dash between the steering column and the inner body panel
Check to see if the vacuum dump valve on the brake pedal is present and working – it has 1 vacuum lie attached and may a have a threated plastic housing and a yellow plunger actuated by the brake pedal arm. It is a tight slip fit in the mount and can be adjusted with some pushing or tugging.
