an aftermarket tach needs the tach driver there is no way around that to my knowlege. it would be much easier (maybe cheaper too) to find a new cluster and just swap it out and you will have a tach, i did this with mine no problems, HOWEVER the mileage on the cluster stays with the cluster. so if your car has 20k on it and the new cluster has 30k it will read 30k and most dealerships wont reprogram it for you or charge you an arm and a leg to do it. try to find one close to your mileage and swap it out.
$85-$125 is alright i got mine off the Buy-Sell-Trade forum here for $40 so keep looking. and sometimes you can find one on ebay for less than that
ZETEC Crew #26 SR #20 YELLOW BLAZE OWNERS GROUP#4
Moderator Pilot, 12 SE HB ╠╬╝, 2011 Ford Edge SEL AWD