The Starter relay is in the Power Box in engine bay, but not sure that is the starter interrupt relay, I would say no.
But I have read that PATS grounds the Starter relay, to allow starting.
What I would do is to remove this relay and test slots 85 and 86 these are the relay coil's power.
With Key on one of these should be a ground
With key turned to START, and clutch pedal in, the other should have 12v
If no 12v then Clutch switch or fuse 24 could be the issue.
If no Ground then PATS would be the issue, no reason you couldn't Ground the relay coil, by-passing "starter interrupt", but you would need to know which slot should be the Ground, 85 or 86.
This relay passes power from a fuse in the same panel to slot 30 in the relay socket, it should have 12v all the time
MAKE SURE TRANSMISSION IS IN NEUTRAL.
If you jump slot 30 to slot 87 starter solenoid/motor should activate
PATS has 3 parts:
Wand or antenna on Key switch, this allows reading of key tags
PATS module, above glove box, this is connected to the wand, it has memory for what key tag numbers are allowed, computer may also store these.
Computer(PCM) it is connected to PATS module and gets the "yes or no" signal for starting, usually just fuel and/or spark, the starter disable is not used on all PATS.
PATS module and PCM are "married" at the Factory, one can not work without the other, and they can not be swapped without "remarrying".
On power up they "handshake" to confirm they are both present and working and that the PATS key is OK or not OK.
If you don't have the Anti-theft light flashing then PATS shouldn't be on line.
The starter interrupt relay may be IN the PATS module, it is just a Ground for the Starter Relay.
PATS info can be as clear as mud on some things, lol, it is an anti-theft device so I guess that would be prudent.
If you hooked up Reverse lights on the manual trans you had to access the NSS connector, I can't say how Ford would have setup the the Main wiring harness pathway for manual and automatic.