Ok I recently got a second mercedes for a steal. 1983 300sd in running condition for $400. Well the tachometer does not work so after doing some googling it points me at the tach amp as being the issue.

At one point I decide to swap the amp from my main car (1984 300d) but when I went to swap I found out the 84 does not have one. I have the plug for one and a cap ontop of it but the cap is empty. Yet I have never had any issues with my tachometer at all. What is going on here?