Here are the tips for '89-'93 ('94-'95 are easy):
The cylinders are numbered like so, facing engine from front of car. Front to back left hand side, then right hand side. #1 is the front pass and #3 is the rear. #4 is the front D/S and #6 is the rear, got it?
#1: Done from up top, remove the air intake tube and put in trunk for safe keeping. Drain and remove the coolant overflow bottle and put in trunk. Now you can get to #1 with a swivel on the spark plug socket and about two or three 3" extensions on the ratchet.
#2: Done from under the pass side. Do to the tight fit it is easier to use a short socket that fits on the hex head of the spark plug socket. Push the spark-plug socket on by hand then slide on the ratchet with the short socket that fits the plug socket head to loosen and install.
#3: Underneath, use a short (3") extension on the ratchet with the plug socket. It may be easier to push the socket and extension on first then put the ratchet on them.
Warning: I may have 2 and 3 mixed up in my memory but I don't think so. If you try and it seems like it then switch methods and see what happens.
4 & 5 are done from underneath the driver’s side.
#4: just socket and ratchet, fits and turns. May have to put socket on first like others.
#5 & 6 are pretty easy.
Do make sure wires get centered then clicked down. Don't try to crimp the terminals for tight fit on tip of plug, for any reason. Good wires have boots bonded to them; with cheap wires you have to make sure the terminals are facing the plug before you install them down.
When installing the plugs coat the threads lightly with anti-seize. This crap is messy so have a roll of blue towels handy. If you get some on the tips clean with carburetor or brake cleaner and do over. Tighten plugs until they just seat then just a bit more and don't strong-arm them.
When loosening old plugs if they are stiff don't strong arm them, just maintain steady pressure and they usually bust loose within 5-10 seconds, if not slightly increase pressure and try again. The engine must be cold (aluminum threads).
If you drop a plug inspect it big time. If in doubt get another one--this is very important. If you start it up and it's missing badly one's broken or defective. Without a scope it's a guessing game as to which one. If this happens and you dropped one or heard some cracking sounds when torqueing one in try replacing the plug(s) in question first. With fresh anti-seize you can do it warm, after it's hot don't touch it!
Study the routing of the wires and only remove and replace one side at a time to avoid mix-ups. Match the old wires to the same size wires in the new set. Take each engine side of the plug wires off while still in the looms; there are three on each side. Then match the wires to that side and lay them out next to the old ones, spaced exactly alike. Then transfer the looms one at time to the new wires. Start from coil ends to plug ends and put the three for that side back on in one piece. Coat the inner boots and coil seals with Dielectric Silicone, also sold as Dielectric tune-up grease. This prevents moisture from getting to the terminals causing misfires and corrosion; it also keeps the boots from sticking. Good wires like motorcraft have it factory applied, make sure you twist all connections to spread it completely around.
Make sure that the new wires sit clear of any hot or moving parts when on. Avoid letting the wires touch for very long, crossing is ok but not running against each other.
I use and recommend Motorcraft AWSF-34PP plugs as they will increase power and reduce detonation at WOT. They can be installed as is for great results; they come with a 0.047" gap. Don't be concerned because they have a shorter tip, this is a good thing. If you can't afford Motorcraft ask for the '94 Mustang 3.8 plug. Whatever plug you choose it has to be a double platinum type due to the ignition system. Under no circumstances use Bosch Platinum of any style. They feature an extremely fine center electrode that is great for Mom and Pop but will melt away in your SC.
Motorcraft wires are your best choice also but a high quality aftermarket brand will work as long as it is made to OEM specs (spiral wound solid metal core). Thicker wires don't mean better spark just better protection. I also like Borg Warner but avoid Napa's Belden wires or Auto Zone's house brand (I don't care how cool red looks or how long they warranty them, I just want good wires that work.)
While you're at it replace the RFI Capacitor that attaches to the coil pack and is wired into the coil power lead. This is a cheap easy to replace part that never gets any respect. If yours starts to fail you'll have a hard to diagnose loss of power.
Vernon