Curious issue with Westinghouse EW300 double oven

[RrnR]

We have a wall double oven model EW300SC*01 about 25 years old.  It's installed in a beach house (which might be part of the problem).  The problem is that the top oven won't run without the bottom one running.  The top oven is dead until the bottom oven bake knob is moved off the OFF position, then all the lights behind the top and bottom oven controls turn on.

This has just started happening.  I have tried different circuit topologies in my head but I haven't come up with one that would cause this behaviour.

Any ideas before I rip into it (I'm hoping there's a schematic on the back).?