1996 Club Wagon XLT: Electrical: Where to tap for tach "pulse" signal

I have a 1996 Ford Econoline Club Wagon XLT (5.0L V8). I'm installing an aftermarket remote starter/keyless entry system.
The control unit is made by Audiovox, model APS-55TW. It needs a
reference for "engine idle" in order to know when to stop cranking the starter motor. The simplest reference would be to measure the tachometer when the engine is already idling.
My question is: where can I easily find a reference pulse for the tachometer? There is no tach gauge in the van, but the vehicle is new enough to have OBD-II which includes provisions for measuring the tach, among other things. Which wire should I be looking for?
Thank you, The Iron Giant
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