Hot tub is an older Jacuzzi from mid-90's - Jacuzzi platinum series, lexus plus model.

The GFI trips as soon as I switch it to the on position. Per the suggestions on this board, I diagnosed that it is the blower. I replaced the blower with a brand new model. Same result, gfi trips immediately. Blower is 110V 1hp. So the problem is not the blower itself.

More backstory - It appears as though a mouse built a nest in the control box this past winter, and chewed through the insulation on the hot and neutral wires that lead to the blower. I don't know for certain, but suspect, that these two wires shorted at some point. So, I'm thinking that the problem was never the blower itself, but maybe in the control box circuitry of the tub. Perhaps the short damaged a component on the main board?

Without the blower motor attached, the remainder of the tub appears to work flawlessly.

Any ideas? How can I go about troubleshooting the problem on the main circuit board? Anyone know where I can find a schematic for a tub this old?

If just disconnecting the blower motor allows it to work properly, those "damaged" lines are still getting power, correct? It is only the motor itself thats being taken out of the loop? If so, I'd say it's a problem with the blower wiring circuit, not the physical wire, or moisture is getting into ther blower and ruining it in short order. Was anything vhanged when you put in the new blower, by chance, maybe, just maybe, you tied into neutral wire on the other (line) side of the GFCI? If you have the hot on one side of the GFCI, and the neutral on the other, it would also trip.

Also, and this is going to sound very strange, have you tried disconnecting both heater leads, and trying the air blower again?