Sounds like the brake light switch is broken or out of adjustment and yes it would cause the cruise control not to work.
There are several ways to fix the cruise control on your Mercedes-Benz depending on the problem with the cruise control. Make sure the fuse for the cruise control is not blown.
cruise control does not work when headlights are on
Cruise will NOT work if the speedometer has a problem
If the cruise control on your 1987 300ZX does not work, you might have an issue with the cruise control fuse or wiring. You could also have a problem with the switch that turns the cruise control on and off.
E62 is a vacuum solenoid problem with the cruise control. E92 is a heated windshield request problem. E99 is a servo apply problem with the cruise control.
If the cruise control will not engage, there might be a problem with the Electronic Fuel Injector or the MAP Sensor. There might also be a problem with the Oxygen Sensor. Some autos have more than one oxygen sensor.
That sounds more like a mechanical problem with the cruise control cable. Check for a damaged or worn cable from the cruise control module or from the accelerator pedal. You should take the vehicle to a shop that specializes in Saturn's, and describe the problem. Don't take it on the highway again until the problem is repaired; in fact, you probably shouldn't really be driving it.
There are multiple things that can go wrong with the cruise control, you need to be more specific as to the problem your car is having. While traveling at highway speeds, the cruise control suddenly deactivated itself and would not come back on.
The cruise control may not work is there is a problem with the brakes. Check to ensure that the cruise lights come on in the dash. Also, check all the vacuum lines to see if there is any issue.
depending on engine size it could be in 4 different places. 2.5 liter is behind cruise control actuator.
First you need to troubleshoot the 1994 LeSabre cruise control. Check the fuse, the cruise switch, the linkage, to find where the problem is. Check the vacuum lines on the engine as a vacuum leak can cause the cruise to not work properly.
A. There is no "Cruise Control Relay" per se. The power for the cruise control comes through fuse #14 in the fuse box. From there it goes to the Cruise Control Switch. However, the Cruise Control Module is above the Fuse/Relay box, behind the dash board. There is a special cruise control tester, VAG 1466, that Volkswagen plugs into the system to pinpoint malfunctions. I think it would be worth the money to just have Volkswagen check the system if you find the fuse is not the problem.