I was having the same symptoms on my 88. I had running lights but no turn signals or brake lights. Here is a pic of my 89 under the dash just above the driverside kick plate.



My 88 didnt have the black box in it when I got it (and still doesn't). The plug to it was just sitting there dangling. I had no idea what the box did or if it even had anything to do with the lights. After I chased the wiring down and couldnt find a reason why it was doing what it was, I plugged a regular flasher into it and they worked. I'm not saying thats what your problem is but its worth a shot. HTH.