We have an ’89 and I have been all through this problem...here is how
it works...when you insert your key with the black resistor, contacts
in the ignition switch read the resistance, and if correct, closes a
relay in the drivers side kick panel called the "security relay".
This is a large relay with some sticky goo tape wrapped around it to
keep it from rattling. Some cars have two relays in the kick panel,
only one is the correct one, you’ll figure it out when you look at it.
Only when the security relay closes will the car start and the
security light go out. In our case the security relay got wet from a
bad body seam inside of the fender, it allowed water from the front
wheel to splash in. The relay can be purchased at Advance Auto Parts
for $13, be aware that some auto parts stores will not have a listing
or even know what it is. I would start there, especially if you can
turn the key multiple times and sometimes it will start, then I
guarantee that the relay is your problem. The other possibility is the
contacts in the ignition switch, but that is a much bigger job and
more expensive. In my past experience, I would go for the relay first.