Tach only works with fuse 9 removed & general electrical problems
 

Car:1989 300e

Problem: A couple days ago, my courtesy lights were staying on all the time (sometimes flashing) so as a temporary solution I pulled the fuse (#9) because I was already at work. Surprisingly, the tach which has never worked in the 12 years I have had the car, began working only when the headlights were on. And now when I replace the fuse the courtesy light problem is no longer but the radio power flashes partially on and then turns off.

Question: Is there a general strategy on how I should go about tracking down the problem? I'm really not sure where to start. I searched the archives and didn't find anything but maybe I was using the wrong search words.