The tach signal passes through a relay that is located behind the battery. Check this thread for some wiring diagrams, pictures of components, etc.

http://www.peachparts.com/shopforum/showthread.php?t=228264

__________________

1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD