yer they do dim.
I had to do a slight mod to the holder to make them fit. the standard fit one is 31mm i think...and there is no way you will find an LED bulb bright enough which is 31mm.....well untill NOW!!
if you search ebay and look for something like 'Festoon Dome 31mm 9 LED (Xenon White) Bulb' you will find the exact bulb that you will need with no mods needed.
I put in two LEDs one facing the back and one facing the front......as you may know the light you get from an LED is directional so that's why i needed two.
Don't even bother with the 6 LED versions..they are no way bright enough if you want them to be useful. I made two holders using paperclips to hold the second LED onto the first one before clipping into the holder.
I did the door ones to and these are the standard side light bulbs size.
I tried the blue ones but they looked abosolutely crap lol.....if you go with the white xenon LED ones they already have a tint of blue.
Hope this helps...the door ones were very easy to do, as was the glove box one lol
here is some inside pics