Ron Dittmer has a post elsewhere (
http://forum.phoenixusarv.com/index.php/topic,2234.0.html) of his addition of a voltmeter to the test panel below the microwave. I just completed the same project and thought I'd add a few comments. His pictures are so good that I didn't post new ones.
The first trick is getting the cover off. As Ron mentioned, there is a sticker under the vent hood with instructions, but if your sticker is missing, here's the trick: First slide it all the way to the right and locate the little notch on the lower edge. Use a screwdriver to carefully pry it up so the latches on the left side are free. Then slide the panel to the left (which frees the latches on the right side) and pop it off.
I didn't buy a fancy unit like Ron's, with a thermometer and a clock. There are several alternatives for sale on Amazon, but be sure the one you choose is a true "two wire" installation, where the power to drive the LED's comes from the same voltage source you are measuring. Most use four wires, with two for power and the other two for the measurement. Also, be sure you order a DC voltmeter in the range of about 0 to 20 volts and not an A/C meter.
Here's the one I used ($6.60):
https://www.amazon.com/gp/product/B00HWJH7RQ/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1The size of the meter was just a hair less than 1" x 1.75". I made a cardboard template, centered it on the knockout area of the panel, and marked it with a sharpie. Then I used my Dremel tool to carefully cut the hole just slightly smaller than the lines. The panel is VERY thin, so be careful. Then I used an emery board to carefully enlarge the hole until it exactly fit. The meter has ears that hold it in.
Since I didn't have temp and clock like Ron, I wanted mine wired so that it only came on when I clicked the "Test" button. That turned out to be very easy. The upper right corner of the electronics board has the 12V power connections. The red removable terminal with the red wire is the negative (ground) side. The black wire soldered to the board just above it is the switched positive (+12V) side. As Ron mentioned, that's exactly opposite to everything else I've ever worked on, so it wouldn't surprise me if they've corrected the wire colors on later units (mine is a 2008 2350, like Ron's). Check it with a digital voltmeter to be sure. The red wire on the voltmeter must be wired to the +12V side.
The wires on my voltmeter were short, so I had to add some length. I just soldered them, but obviously you could use solderless connectors if you're not handy with a soldering gun.
