Here is how to install a keyless lock/unlock system for around $25 and in about one hour. I hunted through forum to find detailed instructions on how to do this on my model car, and only found bits and pieces, so here goes. This will only work on those cars that will lock and unlock all doors from the trunk (later SD's, from what I understand). This makes this so easy because everything you need to do is in the trunk (no messing with the doors or actuators).

1) Purchase cheap unit from eBay. Mine came from China and was about $25. It is imperative that the unit have a signal delay for pneumatic operation. Search listing for work pneumatic, and if not there, don't buy.

2) Locate air pump in trunk. It sets next to spare tire and is easily accessible by removing a cover that is held in place with a couple of screws.

3) Remove the jumper on the wireless module for the pneumatic delay option to allow the 3.5 second hold that the pump will be looking for.

4) Unplug power from pump just to eliminate any chance of shorting something (it is the smaller plug with red and brown wires). You will need to splice the keyless unit into one of the wires (I did the green one which goes to the paassenger door). You will also need to pick up power and ground signals which can come from the red (12V) and brown (ground) wires that feed power to the pump. Follow the schematic I show in the pic. This took me the longest as I could not locate a schematic anywhere that was a perfect match for what I needed. Use the hand drawn one on the right side.

5) I placed the unit right on top of the rubberized pump holder, and underneath the trunk spare tire cover. It fits nicely in there, and you can secure it using some duct tape (or something more elegant). You can trim back all the wires in the unit that aren't needed and it makes for a pretty neat install.

6) Mine came with a blue flashing LED that flashes when the car is locked. I ran that wire up into my taillight, on the clear lens, and it looks really cool at night, and let's you know things are locked up. Takes about 2 minutes.

That's it. Of course, there are several wire connections you need to make using your favorite methods, but it is a pretty clean install.

Some tips:
-I tried to use some scotch connectors from Harbor Frieght for the splicing. The concept is cool, but in practice I was not getting very good connections, so I had to remove these and just go with butt end connectors. Soldering and heat shrink would probably be better.

-If you find things don't work, it might be a bad connection, and you just have to use a multimeter and start looking for continuity and proper ground and power voltage at the proper location on connectors. Took a little while to resolve my bad connections. Do it right the first time and it will save you time.

-The top three wires in the schematic are used for unlocking. The bottom three are for locking. The system requires a ground signal to lock, and 12V to unlock. The three wires merely route either a 12V signlal or a ground signal to cause the pump to have pressure or suction.

-In theory, you could probably splice into the yellow wire (goes to trunk lock switch), or blue wire (goes to driver's door), but I used the green one.

-When you are done, you should still be able to use the key to lock/unlock all the doors, as long as that worked before installing the unit.

Hope this helps someone out there, it would have saved me a couple of hours if I knew before what is in these instructions. Thanks to member locry for the idea to do this from this thread (Door Locks, Vacuum, and Ignorance, 300SD) and thanks to member ah-kay for assistance as well.

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