There are a few possibilities, Make sure the battery is fully charged then check to see that the wire going from the solenoid to the starter is clean and tight. Sometimes the connection gets corroded. If these check out I suggest pulling the starter and taking it to the auto parts store and have them test it. Always disconnect the battery first before you remove the starter.
Wiring diagram of a 1990 Ford Thunderbird Solenoid. I need a picture of which wire goes where on the side wall by the battery
The starter solenoid is physically mounted on top of the starter motor itself.
you need to replace the starter solenoid usually on a ford the solenoid is located on the side of the engine bay
This sounds like your starter solenoid is bad.
Have the starter tested at your auto parts store. Sounds like a bad solenoid or bad starter.
How do you remove calipers from a 1987 thunderbird? Like where do I start?
Click the link.
Ford stopped making the Thunderbird in 1997 and didn't start again until after 2000. So you don't have a 1998 Thunderbird.
You should hear the solenoid click when it closes and is working correctly. If you hear no click it is more than likely defective.
Ford Thunderbird was created in 1955.
Jumper wire from Battery + to Coil+Jump starter solenoid from smaller trigger terminal to starter side
You can view the Ford Thunderbird owners manual online ( back to the 1996 model / as of November 9 , 2012 ) at : www . motorcraft service . com ( no spaces ) Click on Owner Guides