Take the meter out. Look where the factory bulb is connected and connect your LED wires there. Carefully drill a hole into the bottom clear cover of the meter and install the LED in it. Probably have to use two in order to get proper lighting and avoid shadows in/on the meter.
If you’re using a 5mm LED, it’ll be a little harder than if you’re using 3mm. And depending on the viewing angle of the LED, you might benefit from clipping the end of it to make it have a broader pattern and therefore light more of that meter.
If you stick it behind that meter, you’ll have most of the light concentrated in one area on the back of the meter.
You either get LEDs with a wide angle viewing pattern or you stick with the tube style bulb like comes in that radio. I use 5mm and 3mm LEDs, clip the end of the 5’s and don’t on the 3’s. I use my soldering iron to poke a hole for the 5’s, small drill bit for the 3’s. A tad of hot glue will hold either in place and it’s easily removed with alcohol.