Ran across this in a google search, so adding info. Yes, "old 2017 post!" (beat some gomer to that punch so get-a-life). An earlier post in 2007 had incorrect info claiming the window switches were directly powered from the key switch. Not true at least in my 1985 300D. My Haynes manual has a schematic for 1984, which shows 2 relays in the power source for the window switches, with one enabling the other thru the door jamb switch, though asterisked as "optional" for all 3. I don't have the FSM. Perhaps the door switch is a disable for certain models which might have the glass or weatherstripping damaged if actuated when the door is open (2 door coupe or convertible?). If anyone knows, chime in.

Before diving in, the most common problem is a corroded fuse. One fuse powers LF & RR windows while another powers RF & LR windows. So, if diagonal windows don't work, check the fuses. Since I replaced all tin fuses with gold-plated ones, few such problems. Another common problem is the switches get funky, especially if your family enjoys spilling drinks on the console like mine. Usually that will stop a window (or sunroof) going in only one direction. You can remove the switches, pop open by slightly prying the side away from the plastic "axle", and clean the silver contacts, reassembling w/ silicone grease. Work on a clean dining table, preferably in a tray to contain loose parts. That is almost an annual task between my two 300D's.

This time, per post title, no windows worked but the sunroof did. In the fusebox, both a and b fuses had no power, so went to the Haynes and found these WTF enabling-relays but had to google to find where located. I had seen that relay behind the dash cluster but was thinking "flasher" though the flasher relay is in the console and not a standard Bosch relay like this. You can leave the cluster in place and just remove the lower dash panel to reach up and grab the relay, knowing the bugger is there. If removing the dash, I always push it out from behind anyway since the tug-shims people show don't work for me and risk breaking something, so lower-panel off is part of that anyway. I swapped in the relay for the radiator fan and voila my windows work (only one on inner L fender in my 1985, identical AC relay there too in earlier 300D). Now I wonder if my car has the 2nd relay which enables the one I swapped. My windows work with the driver's door open so likely doesn't.

The rest here is general to any of the standard Bosch 30A relays in your M-B and even new ones in recent U.S. cars (flat spade connectors). The terminal numbers are German DIN standard (U.S. too), but the 85 & 86 for coil apparently have no standard polarity that I've ever found. That only matters if they have an internal diode which could be a short-circuit if wrong polarity. You might check first using ohms on your multimeter but use another meter to determine which lead is positive in ohms mode (sometimes black). If you measure less coil resistance with one polarity, use the other polarity to not short-circuit. This M-B relay appears to have a resistor in parallel w/ the coil (per legend on cover) so polarity doesn't matter. One web article found a relay base w/ diode oriented opposite what many state is proper coil polarity, so nothing is assured. To be ultra-safe, you could actuate the relay position w/ relay out (turn key to "run" here) and measure polarity applied by the car on 85 & 86. I didn't worry and just connected 12 VDC across the relay since in "salvaging a relay" mode.

As-found, the relay would click with 12 VDC across the coil, but no continuity between 30 (12 VDC source usually but not always for a subtle reason) and 87 (normally-closed). Also open from 30 to 87a when unpowered (not used anyway). I popped the top and cleaned the contacts w/ 600 grit sand-paper plus spray Contact Cleaner, then got low resistance from 30 (red clip) to 87 (wht) when pressing the contacts closed by finger and then with coil actuated (grn & yel). Ignore the 20 ohm reading since I get that with multimeter leads clipped together (cheap Chinese stuff and/or low battery). Both aluminum cover and base are numbered so easy to orient the cover correctly before crimping it back on. This one will now power my radiator fan and the contacts usually work there (until burned away) since that motor is more energetic. BTW, now that you know how they work, for many functions, you can pop out the relay and jumper term 30 to 87 to force the action. In my M-B, I carry a jumper w/ male bullet connectors for that. In my U.S. cars, a jumper w/ male spades (Packard 57). Usually to jumper the starter motor relay or fuel pump, though neither apply to my M-B, but you could perhaps force on the radiator fan if overheating to make it home. Don't force on the AC clutch or it will overpressure the compressor outlet to perhaps slip the clutch to melt.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans