Just in case you determine that the interrupt rlay is not the cause of your problem, you may want to be aware that corrosion of teh small wire attaching to teh starter solenoid is often a cause of teh symptoms you describe. Ford has issued a TSB dealing with this problem, as well as an inexpensive connector repair kit that most dealers have in stock:
NO CRANK - POSSIBLE CORROSION AT STARTER SOLENOID - VEHICLES WITH 3.0L AND 3.8L ENGINES
1992-95 TAURUS, THUNDERBIRD
1992-95 COUGAR, SABLE
Some vehicles may exhibit a "No-Crank" condition due to corrosion at the starter solenoid
Replace the corroded connector with a service wiring and connector assembly. Applying di-electric grease, along with replacing the wiring and service connector, to the terminal connector will aid in corrosion protection.
DETAILED SERVICE PROCEDURE
Disconnect negative battery cable
Disconnect the christmas tree retainer retaining wire takeout.
Remove the connector from the solenoid by firmly grasping the connector and pressing the external detent release (if so equipped) and pulling. Refer to Figure 1.
CAUTION: DO NOT PULL ON THE WIRE BECAUSE THIS WILL DAMAGE THE TERMINAL.
DO NOT SAND OR FILE THE TERMINAL. Brush Di-Electric Grease (D7AZ-19A331-A, or equivalent) onto the terminal. Refer to Figure 1.
Using scissors, carefully cut the protective sleeving 4" (100 mm) back from the connector.
Push sleeving back and cut wire 8" (200 mm) back from the connector end.
Strip wire 5/16" (8 mm).
Insert wire into butt connector of Service Wire and Connector Assembly (F4VY-14A411-A) and crimp using tool for pro-insulated crimps.
Heat splice with heat gun until tubing shrinks and adhesive flows from each end.
Pull sleeving back over the splice and up to the back of the connector.
Fully tape over the slit and wire up to the back of the connector.
Replace harness retainer and tape it 61, (150 mm) from the end of the connector.
Assemble connector to starter. Insert harness retainer into floating hole on battery cable
Install negative battery cable
PART NUMBER PART NAME CLASS
F4VY-14A411-A Wiring And Connector Assembly AG
D7AZ-19A331-A Di-Electric Grease AM
OTHER APPLICABLE ARTICLES: 94-18-3
Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage
OPERATION DESCRIPTION TIME
942603A Install Wiring And 0.6 Hr.
BASIC PART NO. CONDITION CODE
OASIS CODES: 201000, 201100, 203200, 601300