An idea is cut the green battery cable there on right side of photo now to install a racing battery cutoff switch either hidden using the existing hole in rear of glovebox (battery drain and theft prevention) or on dash or center console if racecar appearance is acceptable for battery drain and safety precaution.
With the heater box monstrosity done my next step would be tackling the wiring harness while its all exposed. Unwrapping the electrical tape to the engine compartment and removing the wires for electrical equipment you may no longer be using and removing that wiring back to fusebox. So much wiring spaghetti in that pic is gonna be fun.