Make sure the answer is YES to either of the following questions. I meant lock/unlock everything. Otherwise you will need an actuator inside the driver's door, that would be more work.

1) Can you central lock/unlock from the trunk with key?
2) Can you central lock/unlock from the passenger door with key?

3) Buy a keyless remote from eBay which supports pneumatic operation. Most do. Ask the seller. Most seller put the 'pneumatic' keyword in the description, not in the heading. Try this.

e.g.

http://cgi.ebay.com/2-FOB-Universal-Car-Keyless-Entry-Remote-System-Kit-/250597424095?pt=LH_DefaultDomain_0&hash=item3a58c53bdf

This install is keyless entry. The existing MBZ alarm function is NOT affected.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.