My '87 190DT is long gone, so I have no car, no books and only my fading memory. Nevertheless, as I recall, the blower speed control is built into the switch, so if that's shorted you're going to have to find a switch. The '87 had an auxilliary fuse box mounted just forward of the main fuse box. This was a black plastic box, and it held a pair of relays and some fuses. I'm not sure if this was present on all models. This is how it looked in my car, the orange relay controlled the high speed fan. The original black relays were internally fused: as they aged, the heat from the fuse could cause the contacts to bridge. The orange relay has an external fuse, and fixed that problem: