Eddie,
There are as many as 8 pins on the back of the tach. The wire from the coil or ECM must go to pin #4. Pin #2 is hot and pin #3 is ground. The others are not used for this style of connection. Then there are three tiny DIP switches by the terminals. They must be set according to number of cylinders. There is also a potentiometer on the back but this is only used with an alternator feed connection. Play with that as a last resort, but remember where it started from. If VDO says it worked, that makes me think it is in the feed line. If all else fails, set it up as alternator fed and run that line. It will be the W,R,or I alternator terminal and it will also go to pin #4.