i have a circuit schematic for the internals of the Tach if you are really interested Gracey, alternatively If you have a a 12V pulse signal from the ECU it may be quite a simple soldering mod on the tacho to get you working. (asuming you have the same pulse frequency as cossie, ie 2 pulses per rev)
The Ford tacho's are shite, If you look at what happens to the signal internally with the amount of filtering they apply, it's a wonder it ever works at all!! indeed at much over 5500rpm I'm not sure how it does manage to work!

Most of the problems the SECS coildriver has is actually more to do with the tacho's being so crap and variable too, rather than the coildriver system itself..

*EDIT* hmm reading yr post again, I'm not sure which way round you want to do? Do you want to use the BMW tacho (and need to adapt the drive signal) or use the Cossie tacho and get it working with your system?