I am not familiar with the 240D, But I just did a retrofit install on my '82 300SD, so my advice is slanted that way.

First, The green/black and red/black wires are nautical (kreigsmarine?), the red is port-left, and the green is starboard-right. You will need to access the fader control, and when you do, you should find a pair of roundish connectors near it, one black, and one white. You will need to carefully disconnect those connectors.

Then, go to any decent hardware, or electronics store, and buy 8 or 10 (doesn't hurt to have extras) connectors of the sort that are used to tap into existing wires without cutting them (unless you already cut them, in which case, you should use crimp comnnectors to re-comnnect them, unless you plan on selling the car with the new stereo, or being buried in it). While you're at it, you should also buy a wire stripping/crimping tool, and 8-10 connectors of the proper guage for the wire from your stereo.

The first connectors I am talking about are squarish plastic, have a single metal connector that looks like a rounded "W" and a locking plastic outer shell. The connector has one inlet that encases a whole wire, and another that accepts a wire end. You don't have to strip any wires, just put the connector over the existing wire, and insert the "new wire" then squeeze the connector with a pair of pliers to make contact, The "W" will cut into the contacts of the insulated wires, and then you can lock the outer shell.

You will probably also have to extend the speaker wires from your "head unit" to reach the fader area. If so, might I suggest you use a short run of 16-18 gauge speaker wire, like the black "Monster Cable". You should be able to find it in bulk at a hardware store, and can have it cut to your specs.
Connect the "new" extension wires to the existing ones first, using the following procedure: The two pair that go to the fader, and not through the connector, are most likely to be the front. The two pair that were disconnected from the connector are most likely to be the rear. And remember the RB/GB code in the first paragraph...

Once the extension wires are connected to the ones in the car, strip the opposite ends. Then, using a common 9vdc radio/smoke alarm battery, briefly touch the positive (red or green stripe) end of each pair to the + on the battery, and the other end to the - on it. You will hear a "pop" from the speaker that those wires are going to. This is ok, and will not hurt the speaker. If you listen to where the pop is coming from, you will know which speaker that wire set goes to. When you are done with all four pair, you will know what wires go where. To keep from being confused, you can use strips of masking tape and a marker to keep track (RF,RR,LF,LR) of which is which.

Now, if you've done it all right, you should be able to use the crimp connectors (after stripping all of the wires) to connect the new extension wires to the ones coming from your new 4-channel stereo. Remember to be careful about polarity. If you are in doubt, you can always twist the wires together, and use the radio's built-in fader & balance controls to test them (front right, rear left, etc).

You can leave the fader plugged in, but you should cover the bare ends of the connectors that you disconnected in the first palce (the roundish ones that CAME with the car) with either heat-shrinkable tubing (shrink it with a blow dryer), or GASP, if you must, electricians tape.

Any problems, questions, traumas (other than personal ones), you can always email me for advice.

__________________

Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."