The switch absolutely cannot handle the aux fan directly. I don't have a spec for your car, but most Mercedes aux fans draw somewhere between 15A-25A. Far too much for any thermoswitch.

The good news (I'm going to hate myself for getting involved here) is that there's really no need for an additional relay. Wiring the new switch in parallel with the pressure switch shouldn't result in backfeeding current. Best practice is for the pressure switch in the A/C system and the new coolant thermoswitch to control GROUND, not power. Of course, if this is a shade tree hack, all bets are off. Maybe I should back off and ask to see a schematic of your circuit.