Thank you for your question. I hope I can help.
I was looking into your concern and this system is pretty simple to test. Looking at the schematics I could not see a purple/white wire going to any of the cruise inputs. However I did find one which is a VSS signal at the vehicle speed sensor buffer. This is a small box located to the left of the glove box and has a 12 pin connector on it. However it the input from the VSS sensor so your speedometer would not work if there was a problem with it.
Do you have access to a test light and a volt meter? If so run these tests.
Test, with a test light, for power on circuit 397 with the cruise switch on as well as power on 87, and 84, when pressing the resume and set switches, all at the cruise switch and at the PCM.
Then, at the brake switch, test for power on the purple wire (circuit 420) the brake pedal released and, using the meter make sure it is 0 volts with the brake pedal depressed. Perform same test at the PCM.
Next test for 0 volts, using a meter, on circuit 17 (white wire at brake switch) with your foot off the brake and 12 volts with pedal depressed. Also repeat test at the PCM.
The last test is the hertz reading on the VSS signal circuit 389 (dark green wire) at the PCM with the vehicle in motion. I assume that the speedometer is working so the VSS buffer should be working however the wire could be open to the PCM. You can also test that circuit for an open between the buffer and the PCM.
Let me know what you find or if you need more info. I have also attached a schematic to help out.