OK, since electric's are not your strong suite, try 'this' before you bother to add the relay... just remove the violet wire from the brake switch and jumper it to a good solid ground, then take it around the block to see if the CC will engage. But remember, now it will not automatically disengage when you step on the brake so be ready to turn it off at the switch if it does work. If the CC works here, you have too much resistance in the brake light wire for the Global unit and you definetely need to cobble in the relay. Like I said, you might want to do the relay in any case. You can pick up a 5 pin relay at most any auto parts store (do not get a 4 pin). The rest is pretty easy... each spade connector on the bottom of the relay is identified by one of the values in my previous post (85,86, etc.) Just connect them like I stated.
You probably did not wire in what the manual refers to as the Orange Enable Output (ENO) wire. If not, waltz down to Radio Shack and pick up a 12 volt miniature L.E.D (green was my choice). Hook up the orange wire to one side and ground to the other. Then go test the CC. When you get up to speed and engage the CC this lamp should light up. It also may flicker when you turn the CC off or on at the switch. Let me know how it acts.
They make a both a 'closed' and an 'open' control switch... can you tell me which one you have?
I know you said you went through the diagnostics but I want to double check that you did the VSS test whereby you roll the car a few feet forward/back and check to see that the LED in the unit flashes.