Cruise Control Question

Greetings all,
After reading all the prior posts on the subject (56) I am now ready to attempt repair on my cruise control, I just purchased a 85 190E 2.3 last Friday, the C/C does not work at all, my question is: The PO had installed a "3rd" brake light he tied into the wiring in the trunk below the latch. Before I track down the problem with C/C should I disconnect the "3rd" brake light unit or just make sure proper bulbs are installed? Will the extra wiring and bulb create a difference in the resistance causing an effect on my soon to be repaired C/C? Or will it have nothing to do with it because the C/C uses the tail light not brake light circuit? I put in a request for price for the C/C Amp. with the Parts Shop as I understand this is the most common failure.
Thanks for running this board! I just love it! Thanks for the input! Mutt