Are you talking about the fan relay, or the radiator temp sensor that triggers the relay? The one that screws into the bung in the driver's side radiator tank?
There have been many solutions suggested for this, most of which have proven not to work. Some while ago someone posted a sender that came from (I think) a Honda that apparently worked, but it required replacing the '88 thermostat housing with the 91+ style that has a hole tapped for a sensor.
The simple solution may be to take both radiators to a radiator shop and have them solder the bung from the old one into the new one.