Which wires need to be shared to get the factory tach to work, car is a 99 i dont know which wire is the input to the stock pcm, and which pin on the bs3 it needs to be spliced into a, b, or c.

 

It's been a while, but off the top of my head, try J3 on the BS3 (purple w/ white stripe) and 12 (DK BLU/WHT wire) on the Blue PCM connector. Double check it, this was off a '00 PCM, but it should be the same. PM me if you have any other questions.

Matt