If your lock is anything like the one on my 115/300d, which I imagine it is, then the easiest thing to do is to pull the plug out of the back of the lock. That immediately isolates one possible problem-the lock. The other advantage is that all the revelant connections are staring you in the face. With a couple jumpers you can see if the two circuits going to the starter system work. You can do 80% of the testing of this circuit from the comfort of the front seat. No need to crawl under the car.