It's a 89 gt third brake light doesn't light up bulbs are good fuses are good also high beams dont work or illuminate high beam on the dash as well can any one give me a basic idea what is causing the problems I'm not an electrician or engineer here thanks
First order of advice: If you plan to keep this car, learn how to work on it.
If you are not mechanically inclined or aren't willing to lean, clean it up real pretty and sell it now. This car is 28 years old and wear, tear, weather and previous owners will have taken their toll. Unless you make over 50K a year, your budget won't stretch far enough to cover, family, home, and a older Mustang that you pay someone else to fix. This is a Mustang enthusiast vehicle and you either work on it and love it, or someone else works on it, drains your wallet and you grow to hate it.
The 87-93 Mustang GT had a problem with the fog light wiring that caused problems with the headlights and the multifunction switch and wiring.
SEE Mustang GT Fog Light Fix to fix the foglight problem. The stock wiring isn't up to the job and is overheating. The headlight switch & turn/multifunction switch are affected by Ford's wiring problem. Sometimes it overheats so badly that the plastic shells of the wiring connectors start to melt. This will show you how to add a relay to the fog lights to relieve the overload on the headlight wiring.
Check both the headlight switch and the multifunction switch for evidence of overheating, melted wires and damaged wiring connector shells. You local auto parts stores may have pigtail replacement kits for both switches
Sometimes the multifunction switch that works the windshield wipers, high beams and turn signals gets damaged by the heat generated by the bad connections.
3'rd brake light - you'll need a Multimeter or DVM to check this out.
Home Depot, Lowes or any of the auto parts stores will have what you need. If there is a Harbor Freight Tools near you, they have a $6 el cheapo meter that will work if you are short of $$.
Remove the brake light bulb (and the socket it fits in), inspect it for corrosion or heat damage.
Have a helper or find some way the keep the brake pedal pressed so that the brake lights come on.
Set the meter on the range where it can read 12 volts, Ground one meter lead on the metal striker on the car body trunk latch. Check for 12 volts on the red/lt green wire; no good 12 volts and the wiring is probably damaged at the point where it crosses from the hatch to the body.
Checking the hatch to body ground: If there is a bad ground, the brake light won't work
Set the Multimeter or DVM on low Ohms (Ω symbol) and measure the resistance from the bare metal on the hatch to the metal striker on the car body trunk latch; You should see less than 1 Ω.
If all this doesn't make sense, here's some educational help...
Automotive circuits are mostly simple stuff: a power source, a connection path, a control device, a load, and a ground.
The battery/alternator is the positive power source.
The wire and fuses are the connection path.
Control devices are switches, relays and sensors.
A load is a light, motor, solenoid, relay coil or heater element.
In automotive circuits, grounds are the return path so the electrical power can flow from the load to the negative side of the power source.
Electricity flows like water:
Voltage is like pressure,
Current in amps is like volume,
Resistance is like the kink you put in a garden hose to decrease the pressure or volume.
Power is pressure multiplied by volume or voltage multiplied by current (amps)
Digest that, and you just got the first 3 days of Electricity 101.
Use some jumper wires (connection path and ground) to hook up a switch (control device), a battery (power source), a light bulb (load). Now make the light turn on and off with the switch.
That's the electrical lab for the first week of Electricity 101.
For free automotive electrical training, see Automotive Training and Resource Site . Once you are there, select online instruction. I have personally reviewed the material and it is very good. If you are new to automotive electrical troubleshooting, I highly recommend you spend a hour or so going through the material. You'll save at least that much time troubleshooting problems.