Well, stock Ford units are current sensors, ie inline w/ the resistance wire connected to the postive side of the coil... did you make sure to hook it up like that? If that doesn't do it, are you sure the tachs are any good? Of the 2 I've had experience w/, 1 was completely bad and the other needed restoration. However, if you are sure you have good tachs that are wired properly, I know some manufacturers make a little part (the name of which slips my mind... DOH!) that hooks between the + and - coil terminals to provide the tach w/ the signal necessary to make it work. Good luck!
-Jason
'72 Mach 1 || '90 Talon TSi AWD
http://nosaj122081.tripod.com/parade3.jpg
"Thats not a leak, my car's marking its territory!"
"If you've done it, it ain't braggin'." -Roy Rogers