I realize this may be way too late for you, but it might help some others.
This stock GM/CHEV radio can also be found in vehicles such as Silverado, Sierra, Tahoe, Yukon, etc.
There are 3 bulbs that are 4mm, and 7 bulbs that are 3mm I believe. 10 altogether. It's all 12VDC and they're all in parallel.
I used 12V, 5mm LED bulbs for all 10 of them but it didn't go so smoothly where the 3mm bulbs were. Mostly because the 5mm legs were a bit too thick and wouldn't pass through the board so I just had to keep them short and solder them on the front of the board.
Using LED's for the radio is also tricky because it ends up giving you hot spots and uneven lighting across the entire faceplate. The row of buttons directly below the display ended up clearly dimmer than everything else because it's all lit by just 2 bulbs (5mm) which face the back of the black plastic faceplate cover, and there are a couple clear diffusers to spread the light across. Problem is most LEDs are fairly directional so the light doesn't spread well to begin with, whereas the stock bulbs don't have that problem and can also reflect off of the white cover on the front of the board.
The row of buttons directly below the display still lights up well enough, as much as stock if not a bit brighter, but the rest of the buttons, especially the volume and tune nobs, as well as the bottom row of buttons are far brighter. Maybe if I had matched up the LED sizes to the stock bulbs it would've turned out better perhaps. Depending on how particular you are, you may want to be prepared to have to figure out a way to brighten, dim, or diffuse some of the LED's to get even lighting across the entire faceplate. A few of the LED's are facing directly into a button all by themselves so no matter what size they are, they're going to be bright and create a hot spot.
Here's a link to a YouTube video which has a bunch of useful info in it that shows a complete teardown of this radio, along with the bulbs being replaced. It's stock bulbs being replaced with stock bulbs however. They don't install LED's, but it would give you a much better idea of what to expect.