My 240D had a cassette tape holder with these junky spring-loaded chambers that I think may have come from the factory but who needs tapes, so I ripped out the tape holders and built a wooder pair of cup holders out of it... Perfect.

But to possibly make a considerable contribution to the group, how about a hydrogen supplement to the system? It works pretty great and I will take some pics of what I've done if anyone is interested.
It's basically a simple hydrogen electrolyzer in the trunk, made from a plastic paint can, that converts salt water to 3psi of hydrogen right off the battery power. It's a very un-intrusive mod that can only be noticed as an extra tube and power wire under the hood and the same in the trunk.
At the moment I have it relayed off the subwoofer signal from my stereo because I had already run a wire for a butt-kicker sub under my driver's seat before I began this project. (I guess that counts as a mod as well.) This keeps it from running the battery down when parked (and also makes me have to drive with my stereo on if I want to experience the benefits... which it is always on anyway.)
I have tried two destinations for the hydrogen gas feed; one directly to the air filter housing to dump the hydrogen in with the incoming air, and more recently I built a small brass fitting assembly that injects the hydrogen directly into the diesel fuel right after the secondary filter.
Total price for all the parts was around $35. Took me a couple hours to get it all hooked up, which would go a bit faster if I were to do it to my wife's 300D.

What I know is that I was getting 29-30 mpg before I started, on Michelin X all-season radials. Hooked up the air-filter feed version of the system the same week that I had to change over to snows for the winter, and noticed a significant difference in acceleration and highway cruising. Unfortunately, any mpg rating at the moment would be heavily distorted because of the snow tires... So I don't have empirical data yet on this.

Then I disconnected the feed for two weeks to get used to how it used to drive, to be sure I wasn't kidding myself.

So this afternoon I hooked up the direct-to-fuel version (would that be fuel injection-injection?) and also noted a change in acceleration and cruising immediately. I still have plenty of examination and tweaking to do on this method though, to try to get the best possible results. If I had to guess, I'd say the air filter version had a better immediate result than the fuel-injected version.

At first I was thinking that I'd like to get the hydrogen directly into the fuel because some of it would burn on contact with the air as it traveled into the manifold, making the amount that actually gets to the cylinder less than what I sent in.
I was especially convinced that the air filter method was suffering once I found out that there is not always vacuum in the air filter housing. In fact, more often than not, there's positive pressure. Not what I expected from my years of dealing with gasoline engines. So my guess is that hydrogen enters the filter housing (directly over the hole that drops into the manifold; it bypasses the filter) and then must swirl around a bit in the semi-vacuum that exists there. No good for a gas that begins to combust with its first contact with air.
So I am still learning what's the best method but I can tell you it's easy to build, easy to install, and definitely exhibits a difference in drivability.
I can describe the process I went through to come up with my solution if anyone expresses interest; again with photos that I have not yet taken.

And thanks to the group for helping me learn that I had to take two screws out of the side of the auto transmission my wife's 300D to get the vacuum modulator out. Can't see much up in there, even with the driveshaft disconnected and the rear of the tranny dropped a few inches! I hope to be able to make some sort of return contribution in the years ahead.

Best regards

__________________
Garrison R.
'83 300D