geen:

A couple of misconceptions may be hampering you.
1) The aux fan is not wired to run constantly with the A/C on.
2) The switch in the receiver/dryer is not a pressure switch; it is a temperature switch.
3) Neither of the temperature switches (coolant or refrigerant) carries power to the fan; they simply cause the fan relay to close.

Here is what you do know at this point: the fan does work, the relay does work (because the fan will run), the wiring from the coolant temp switch is good (fan runs when wire is grounded).

To be determined: condition of wiring between refrigerant temp switch and relay. Remove both wires from refrig. temp switch, and bridge the two wires. Turn key to ON, fan should run.

In the event that all the wiring is good, you are left with these questions: is the coolant temp switch good (100C)?, and is the refrigerant temp switch good (62C)?