I have taken some pics showing how the approach light on each mirror is removed. Apart from giving access to the approach light, this will also allow you to clean off most of the corrosion crud around the mirror mazak castings. Believe me, there will be a lot of it ! To fully clean, you also need to take off the mirror back which has been covered elsewhere, and is very easy, use a blunt wide blade at the bottom of the cover between cover and mirror body.

Now, here's the rub. The approach light bulb is not theoretically replaceable separately because the lens is glued on to the moulding holding the bulb. However, with care, the lens can be prised off and the glue seal broken and the bulb renewed, but you'll need some plastic model glue to put it back on again.

1 - approach light holder in the mirror plinth. At first glance in bad light it is not obvious there is a holder at all. The approach light unit fits into this from the top. The holder has to be removed to release the approach light moulding, it cannot be removed on its own.
2 - index finger in position to push and release the holder by pressing in direction away from the door.
3 - holder released
4 - holder being withdrawn rearwards (with care !)
5 - the released holder showing the approach light moulding sitting in it. Approach light moulding is fitted from the top, and released at the front end