OK, straight from Ed's Isuzu Archives (think it came from the long lost lamented ITOG original forum, pardon the formatting):

""I have replaced the bulb twice in the last two years. The second time I used
my mulitmeter to find that the light gets 4 volts from the radio. I went to
an electronics parts store and purchased a "grain of wheat" bulb, 3-6v, 25mV,
for $1.19( I bought an extra for next time). The light is very bright.
Maybe a 6-12v bulb would generate less light. Anyway it works! Good luck
everyone!
>I was recently scanning through the messages when I found some info on
>replacing the backlight on the original Isuzu Tape Deck. Well today I tackled
>that project and would like to RE-VISE the parts needed. Originally I used
>the Radio Shack 272-1141A 12v 25mA bulb as was mentioned in previous posts. I
>noticed that 2 other bulbs had also blown that lit up the pre-sets and volume
>knobs.
>
>When I proceded to test the LCD I was severely disappointed! Very very dim.
>Not up to spec at all. So I went back to the Shack and found a pack of 3
>bulbs in one: Part# 272-1099 which are 12v 60mA bulbs (more current). They
>come in yellow, blue and red. I scraped off the coloring on 2 of the bulbs
>with a knife and put BOTH behind the LCD, hot glueing them to the 2 tabs that
>are there. Replaced the other 2 bulbs with the 272-1141A bulbs which yielded
>good results for that area (basically for just night viewing).
>
>MAN is it EVER BRIGHT behind the LCD!! Not TOO bright, perfect. I do want to
>thank all those who responded to the original post because without reading
>them I would have not guessed that it would be something serviceable (Thought
>the LCD was defective.)""

HTH, I did one of mine a few years ago, and it was VERY bright. You might want to experiment and dial it in. Be sure to use colored lights or pull the little green sleeves off the old lights and slip the sleeves onto the new lights, as plain white looks funny and it too bright.