OK, if you want to replace the bulb behind the air-con switch it is different to the bulbs that light up the heater controls. Of course, it only lights when you have the fan on as well.
So, switch the fan on and turn on the air-con.
Now, I have no affiliation with the seller on E-bay. I just wanted to find a replacement bulb after lots of searching some red twist-in LEDs that work. These just go straight in, remove the old bulb by twisting and then just put one of these in. Now, if it doesn't work, just remove it, turn it 180 degrees and put in as the LED types need to go in a certain way. I marked the back with a black felt tip pen so I knew which way it was previously when it didn't work.
Here is the link to the e-bay seller. These are red, the old one had an orange cover nut I was happy with the red
https://www.ebay.com.au/itm/29321448...0AAOSwLnBXW7dh
Or search for this: 10 x RED T3 8MM Neo Wedge 1 SMD 1210 LED Car Bulb
HVAC Climate Control Light 12V