It looks like they bypassed the thermo-switch on the R/D with an simple switchover relay. When it sees +12v across pins 85 and 86, it closes the connection from pin 30 to pin 87.

I assume you mean temperature and not pressure on the 300TE fan operation.

In normal operation, the temp switch for the aux fan would be open, then the temps would rise and the switch would close and allow electricity to pass through to turn the fan on. With the relay, as soon as the compressor comes on, the relay closes and turns the fan on.

If yellow went to ground, that's probably 85 or 86 on the relay.
I'll guess and suggest the black wire goes to the upstream side of the R/D and normally sees +12 when the system is on. It will probably go to the 85 or 86 (whichever the yellow isn't).
If that's correct, when black gets +12, it'll close the relay and connect across from red to blue. Red would go to the "downstream" side of the R/D and tell your fan to turn on (I'll guess pin 87). But it needs something to say, so it probably gets +12 from the blue on pin 30. The blue on pin 30 was connected to the 260E. Is that wire in the 260E constant hot connected to the battery, or switched +12v with the ignition?