Where can i buy a instrument cluster for a 1979 ford thunderbird?

The 1st thing I would do is check the connectors to the instrument panel.

There are three fuse's that supply power to the instrument cluster !

They are located in the central junction box , do you know where that is located ? Behind dash panel left side .
F2.36 - 7.5A , F2.41-7.5A , F2.61-7.5A What problem are you having ? A gauge is not working , lights not working ?

You have a short in the wiring of either the instrument cluster or wires going to it. If it's the gauge lights you're talking about, most likely the short is in the instrument cluster. Hit the dash firmly but gently enough you don't cause damage, the lights should come back on. The only way to actually fix the problem is to remove the instrument cluster, locate the bad contact with an ohm meter and fix the connection or connections that are faulty.

You need to remove the instrument cluster and replace the special high output back lighting display bulbs, there are 6 of them I think, about $3 each from the ford dealer

Usually a problem with the mini-lights inside the cluster, the circuit board or the dash light dimmer switch.
You can take the cluster out and see if the bulbs can be replaced or check the dimmer.
Not connected to automatic dimmer headlights.

There are 6 back lighting bulbs behind those digital displays. As they burn out the displays get dimmer and dimmer till they go out completely.
You first need to disconnect your battery, then remove the plastic dash piece surrounding the instrument cluster and the 4 screws that hold the cluster in place. There is a "light bar", a plastic strip that contains all 6 lights that can be removed from the side of the unit. Replace with 6 new bulbs in and you should be good to go.

Hi, this is a pretty common problem on these Silverado instrument clusters. We can repair it for you or provide you with the parts to do it yourself. Click here to learn more about these silverado instrument cluster problems: Dr Speedometer Instrument Cluster Repair Speedometer Repair Guage...

Checked Every fuse in the fuse box (no small task).This includes #4 - Power point Instrument Panel, #14 - Day time running lamps (auto lamps doesn't work nor does auto lock), #27 - ignition switch run feed, #35 - Instrument Cluster, #41 - Instrument Cluster, #42 - Delayed Accessory, #45 - Ignition switch run/start feed, #102 - Door locks body security module, #103 - Ignition switch, #110 - Ignition switch, & #116 - Ignition switch. But I also check all other ones as well, they all register OK. AC panel & radio work. Turn signals work. Mileage does not work, trip computer does. So, basically, everything works except auto head-lights, auto locks, and the dash cluster (no MPH, RPM, mileage display, Fuel, Temp, Tran Temp, and Oil Pressure).

It could be the voltage regulator for your instrument cluster is the problem.

fuse maybe?