There have been a couple threads regarding the need for a tach adapter when installing an aftermarket item that requires a tach signal like a tachometer or an RPM activated switch or a shift light. Of the threads I've been able to find mostly mention tach adapters that require splicing into the tach signal wire located in a giant bunch of wires that goes into the fusebox under the hood. Not for the faint of heart if you've never done that sort of thing before.

I found I needed one of these tach adapters to operate my FJO racing TPS & RMP activated switch for my nitrous oxide system as well as for a operating a shift light. After much searching I decided to use the FRPP tach diver. It uses quick taps to splice into the crankshaft position sensor wires which are much easier to get to than the tach signal wire other units require. Everything worked perfectly the first time once I had this unit installed.

http://www.fordracingparts.com/parts/part_details.asp?PartKeyField=5454

FYI-Tapping into the coil-on-plug trigger wires, the fuel injector trigger wires or the tach signal will not operate most RPM activated components. *POSSIBLY* even if the manufacturer says they will. (FJO racing said any of these would work with their device. NOT TRUE. Shame on you FJO Racing. Great product, not as good tech support.)