I did somewhat of the same job for a friend and the GFI was tripping as well. Turned out the heater element had burned through the box it was in and had rusted the control circuit box. Ended up buying a new spa pack. Then found out a pump had seized. Friend decided to can the whole project. Just glad I didnt pay for the wire! Also - The spa I worked on had some sort of ionizer that was hooked to a relay that let the smoke out when I turned the breaker on. Never put money into a free spa off craigslist!
~Matt
