I'd start by doing some more checking. It's possible you still have a ground problem. I would check all the ground, make sure they're are all clean and rust free.
First off safety : before doing this next step. Car needs to be in Neutral E-brake on and you Not in front of the car.
One good way to check the starter, is when it you have a no start and a good battery, is reach done to the starter, take the small connector off the starter, find a piece of wire and a female spade connector, crimp the spade onto the length of wire, put the spade on the starter terminal, take the other end of the wire and touch it to the positive terminal. The starter should engage, if it doesn't it could still be a bad ground or a bad cable from the battery to the starter.
If the starter turns over, you need to check, The starter relay, Theft Deterrent Computer, wiring, etc
A good technician should check all of those when they get a car in with a no start.
There is a lot involved.
Here is the Diagram for Starting from a 89:
http://www.cygnusx1.net/Supra/Librar...x?S=Main&P=038
And the starter is cake to replace. You just need a couple wrenches, a hr or so, and a beer or two
Let us know how it goes