Carbon conductive grease can be found at either eBay or Amazon; search " carbon conductive". I would guess the easyest to find is MG Chemical's # 8481 or 847.
Again I hope you will take your car to an Independent garage and get those BAS/ESP codes read. Most OBD II scan tools will not dig into the specific Mercedes systems codes far enough to glean the information you need. That is why I strongly suggest going to an automotive shop and get those codes read. It will help diagnose trouble in the BAS/ESP system. You can tell the shop your car is basically a 2004 Mercedes SLK 320. They will know what to do from there. DO NOT get a verbal feed back only: Be specific and be certain you have them write the specific codes on a piece of paper for you. TOO much is lost in verbal interpretation and shop BS. Get the codes.