The one major problem I have with the Thunderbird, or any other Ford for that matter is the Tachometer.  The factory tach simply is not in the least bit accurate.  It will tell you where the engine WAS, but will be off by a couple of hundred RPM, even at steady state, and as much as 1200(!) RPM under acceleration.  This simply is not acceptable, but there is a solution.

   When I added the 5 speed to my car, I decided a real tachometer was in order.  I went with the Autometer Phantom 5 inch shift light tach.  You may decide to use a different tach, but the install is the same.  The wiring directions will work with the 94-97 Thunderbirds, but for earlier versions, the only difference is you need to figure out which wires to splice into.

Mounting is easy, as you can see.  It is in line of sight where it is, without blocking any view.  The shift light makes life really easy at the track.  I hit 5200RPM and the light comes on, very visible in my eyes, without looking at the tach, and time to shift.  I added the optional LED shift light, the tach came with a giant light.

To start the install, remove the guage cluster from the car.  This is done by removing the plastic cover under the steering wheel, then the bolts  that hold the black mask in place.  There are also two screws in the top of the mask above the cluster.  The CAREFULLY pull the black mask foward yurself, it will unsnap, but it will require some pressure.  Do not completely remove it once it comes loose, you will also have to reach behind it to unclip the rear window defogger switch from the harness.

Place the plastic mask aside, and remove the 4 bolts holding the guage cluster into the dash.  Do not remove the plastic lenses, there is no need to remove that from the cluster.  Now reach behind the cluster and unlock the harness plugs from the cluster.  You should now have the guage cluster in your lap.  Now would be a good time to replace any burned out bulbs in the cluster.  Also, if you plan to change the guages over to white faces or anything like that, this would be a good time for that as well.  Place the cluster on the side, carefully.

Now we get into the meat and potatoes of this job.  You will notice two harnesses in here that were plugged into the stock cluster.   We will be splicing into these.  One is the C250 plug (grey 18 pin), the other is the C251 (also grey, but 14 pin) plug.  The pins are numbered making life easier.

Now, what you want to do is mount the tach.  I mounted mine on the A-pillar, by removing it from the car(held on only with hidden clips...just pull it off), after measuring and marking exactly where I wanted the tach, then drilling the holes and mounting it with nuts and bolts, combined with blue Loctite.  Install the mount bracket and one of the two screws holding the band clamp around the tach, and re-install the trim.  Then mount the tack onto the bracket. 

Now, take the wires, and route the thru the side window defog vent.

Take the wires from the vent, thru the dash, and they will be inside, right where you need them.

Now we need to connect the wires.  This is where we need to be careful and be absolutely certain of where to connect them.

Splice the red wire from the tach, which is supposed to be hot with the engine running, into the pink and orange wire, pin #17, on the larger connector on  the right side of the wheel.  I cut the wire,  slide heat shrink tubing over everything, then twist the wires together, solder them, then use a heat gun to shrink the tubing.  This creates a permant, live forever connection.  I never use crimp connectors, and avoid at all costs the little blue things that allow you to tap in...they have a nasty tendency of melting and starting fires.

Now take the green tach wire.  This one is the signal wire, the one that actually "tells" the tach how fast the engine is running.  This one will splice into the same larger connector, into pin #18, a white and pink wire.  Again, cut the wire, twist the three wires together, solder them, and shrink tube it.

Next is the ground wire.  This is black on the tach.  On the large connectorm this is pin #s 4 and 6.  They are bridged together, making this easy.  Cut the bridge, splice your wire in, solder it, shrink it, and you are done.

Now, this is all nice, but we want the tach lit up, right?  And it would be really nice if we can dim the tach with the rest of the dash lights, wouldn;t it?  Well, this is also easy.  The tach wire for this is white.  Splice this wire into the SMALLER connector, the one to your left.  The wire you need is Light blue with a red tracer, and is in pin #14.  Cut and splice, and you havee  lighting.  Now re-install the dash, reconnect everything, and re-install the masks. 

You are now done and ready to accurately read engine RPM.  And if you got a shift light tach, like I did, you can watch the tach while you race, rather than worrying about the RPMS to shift at.