I just installed a keyless entry system in my 89 420SEL. I used the system shown over on the W126 coupes web site.

http://www.mbcoupes.com/audioalarm/alarm/keylessentry1.htm

Great price & it works as it should. Only one minor difference in my 89 SEL's wiring harness for the pneumatic lock modual compared to the 85 SEC's.....the red/black wire described on the
SEC is red/white on my SEL. No biggie, just thought I'd share.

Thanks Jim Ham for supplying this info for all to see!
Now I don't have to worry about scratching my doors with the keys anymore.

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73 220D
08 997 C2S
03 Jetta wagon TDi