My 89 n/a left me stranded this morning. A day or two ago I was driving when the following dash lights lit up: battery light, brake light, and tailight indicator. The voltage was hovering just below 12 volts. This morning the voltage dropped to about 6 volts and I had to shut it down. I am thinking the alternator is bad. Any suggestions would be helpful.
Thanks!

Pull the alternator off and get it tested. If it's no good, pull it apart and check the brushes. If they're bad, spend $5 to replace them.

You'll also want to inspect the slip rings if the brushes are bad. It's not uncommon for them to get shaved down by shot brushes.

This goes straight to the slip ring details, but if you look at the previous and next pages you'll find all the details on rebuilding and testing the alt.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=CH&P=10

While the car is running disconnect the negative battery cable. If the car stops running then it is a problem with the alternator, and needs to be tested.

No, that's incorrect. Any car (except for a diesel) will stall if you disconnect the battery while it's running...