I had this same problem when I replaced my starter. I followed the directions below and you can even get the diode at radio shack.
I got this off the powermaster website:
What do I do with the wire that went to the ‘R’ terminal on the original starter?
In early original wiring harnesses, the ‘R’ circuit was a ballast resistor bypass. This terminal is ‘no connection’ when the starter is at rest, and is +12VDC while cranking.This circuit provided +12VDC to the ignition coil during cranking for easier engine starting. Cars that do not have a ballast resistor (i.e. HEI, MSD, or other aftermarket ignition systems) should not need this connection. In most cases, this wire can be eliminated. If the engine has no ignition during cranking, then the wiring of the coil is going to require an ‘R’ terminal signal. To accomplish this, connect a 3A/400PIV diode (or Powermaster part # 600 ) in line with the MOTOR SIDE of the solenoid. (Note: This is the terminal on the solenoid which has the cable from inside the starter motor connected to it. It is opposite the BATTERY terminal on the solenoid. The cathode or banded end of the diode goes away from the starter. This allows current to go from the starter to the coil and yet not from the coil to the starter.)
See
http://www.powermastermotorsports.com/faq-starter.html