I would buy a good quality relay and add a flyback diode.

Also, disassemble the failed relay and look inside. Either the coil fried, or the contacts failed...

We've over complicated a simple wiring setup. You only need one relay, a constant 12v, switched 12v, and ground.

Power the relay on terminal 30 and 86 (switched), 87 goes to fan +, fan ground to chassis, dash switch and thermofan switch provide ground to relay terminal 85.

The dash switch and thermo fan switch will have one wire each going to ground. Essentially all you are wanting to do is provide a ground for the relay with a switch, or two in this case.