From an old post I responded to a while back....
Before you go out & replace your starter, if your fuses and relay check out okay, you might want to check your ignition switch. A bad ignition switch could cause the same problems you are having - your car not starting. An intermittant electrical short in the START portion of the ignition switch will make you think that your starter's gone bad when in reality it hasn't.
Remove the steering column cover & test the switch using either a multimeter or a 12 volt test light, (
http://www.dynolock.net/Test%20Light/testlght.htm)
On 95-98's you can replace the electrical portion of the switch, (a $28 dollar part), without having to replace the entire ignition switch. I'm not sure if this part is replaceable on a 89-94 S 13 - I haven't had to fix one on a 89-94 S13 yet.
Anyhow, hope this helps...good luck with the repair.
ID