That is a 22,000 ohm resistor for the IDM (Ignition Diagnostic Monitor) circuit that tells the computer that the PIP sensor is working correctly. Note that the diagram below shows it's approximate location.
Diagram courtesy of Tmoss & Stang&2birds
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.
Should be along the firewall. I cant remember if the circuit from the TFI to the coil or the circuit coming from the coil to the tachometer has the splice in it. But its typically by the firewall, somewhere in the vicinity of the vacuum tree.