This has come up at least twice and on two completely different models/control boards (after re-installing a newly rebuilt control), but for exact same reason.
First the Symptom Description:
1. When using microwave only the magnetron comes on.
2. When you turn on the oven light the microwave light comes on.
3. When you engage the oven "DOOR" is periodically displayed, microwave light comes on and turntable turns.
OK, so on the first type: model RMC305 / control board 4448877 wire harnesses that connect to board connectors P7 and P4 are swapped (4 pin molex).
On the second type: model KEMC377K / control board 4453377 wire harnesses that connect to board connectors P8 and P3 are swapped (4 molded spade connectors).
Here's what it looks like from a schematic point of view:
4448877wiringX.png 229.78KB 6 downloads
Here's the back of the two different control board styles:
4448877X.png 97.2KB 7 downloads