Beauty, A Clarion is a good choice. I had one for a while and the quality of the sound was excellent . Clarion also make good speakers. I still have a set of Clarions in my car as well as a set of equivalent Kenwood units. The Clarion's are much better.
Anyway, the bad news. The stock Fairmont HU has the ability to turn on with out the need for the ignition key. The Clarion HU (and most others) only operate when the ACC power is on (need the key for this).
There should be 16 pins in the back of your Clarion HU. When viewing the back of the HU, the top row is pin numbers 1 to 8 (8th is a wider pin). Bottom row is 9 to 16 (16th pin is also wider). Pin 13 is the antenna turn on wire.
This wire colour should be blue with a white stripe (on the wiring blocked supplied with the Clarion HU) and this should be connected to the red with blue stripe wire on one of the male two Ford harness blocks (the bigger of the two blocks). If you look at the Ford block with the lock button facing up, this should be the 2nd pin from the left on the lower row of pins.
The best thing to do is use an AERPRO harness to suit a Clarion (Part No CLA002) or an AERPRO universal harness (part No. AP7901F) with its wires soldered to the wires from the male 16 pin block supplied with your Clarion HU If your HU did not come with a wiring block, best get the CLA002. Go to www.aerpro.com
and have a look at these. The universal type is in stock at most SuperCheap stores. The CLA002 ia not normally stocked so they will probably need to order one in for you.