You're going to love this: You have to remove the microwave and replace it from the top! That's right read it again. I just did it on the GMC305, what a joy. This is from memory, so my nuber of screws may be off +/- 1, but here's what you do:
First and most important flip the breaker off that powers your range/micro combo. I like to do this a day in advance of doing maintenance. If you don't have that kind of time get the instructions from the website for de-energizing the capacitor.
Get a set of square drive fittings, exchangable tip screw driver and a good light.
Remove the plastic grills above and below the microwave. The one at the bottom (held on by 4 screws) will come off easily and it helps to open the range door to slide it over the exhaust vent. The one at the top (held on by 3 screws) also "snaps" in on each end. Just flex it in the middle and it will pop out.
Find the two black screws at the bottom of the microwave (where you just removed the plastic grill). These are the only things holding your microwave to the range. Don't remove the two outermost screws that are holding the microwave supports to the range. If you do, the supports can move and you may have difficulty getting them lined back up, it's not the end of the world but it certainly adds to the frustration.
Now slide the microwave out. Sounds easy enough, but it will take some shimmying. I found success with working at the left side first. It will feel like there is something else connected, but just keep at it and it will come. Don't grab the door and yank on it, if you spring the hinges you're screwed over a $2 light.
When you do get the microwave dislodged, slowly pull it forward. You will NOT be able to remove it entirely, because the wire bundle (on the right) feeds up throught the frame and only has a disconnect at the very rear of the unit. You won't need to pull the microwave out that far.
You'll see the wires leading to the bulb at the top center of the microwave. I think it will be obvious how to replace the bulb from here, but just in case. There is a sprung wire holding the bulb in. The right side has a slot. Push each side of the wire until it passed through the slot, that will release the entire assembly. The bulb pulls out of the fitting. Before you replace it be sure it is burned out. Mine went out but the bulb is good.
I replaced the in-line fuse which fixed my door fault problem and allows the micro to operate, but I am still troubleshooting the light situation.
Goodluck! | 
