Good luck.

I rapidly learned to hate older millivolt heaters -- you could fix what was broken, but then still have problems. The available current was so low, that any corrosion on any of the connections or terminals tended to cause a problem. And, when you mix chlorine fumes (from the ubiquitous trichlor) with wiring, you ALWAYS get some corrosion.

Working your way through wiring snaked around inside floppy sheet metal, cleaning terminations did not pay well (since it was usually a do-over) and almost always ended up in cuts on my hands.