I would not suggest disabling the ignition or fuel pump. If something happened to the alarm while you were in traffic, the results could be disasterous.
I have an alarm that is somewhat more advanced than the Scorpio which also has a disabler output. I wired mine to disable the starter button.
Everything I needed could be found on a 10 pin connector under the left side lower cowling - I took power directly from the battery. This connector has the wiring from the right bar controls - so you'll find connections for the kill switch, starter and hazard switch - all you need with the exception of the main power and ground connections.
The colors of the 9 wires on the connector are:
Red-White – Accessory switched power feeding the engine kill switch
Blue-White – Starter button
Dark Green – Left flasher
Brown
Red-Blue
Black
Brown-White
Chocolate – Right Flasher
Green-Yellow
Watch out for the chocolate and brown wires, ya gotta wonder why they'd put two so similar colors into a single connector.
As to the prox detector - under the seat is probably best, but my alarm doesn't use one so I can't comment.
Here's a clickable picture of the connector: