My experience with my L235 was that I also thought the indicator was not working properly because it would never, ever show any signs of heating up. Chalked it up as to the age, 30 years. Decided to replace all of the glow plugs , as well as the indicator. When parts arrived, I installed the new plugs, which was rather easy. Decided to put off installing the indicator because getting to it to change required a lot more labor than what I wanted at that time.
So, naturally, I gave it a try to see if there was any difference. To my amazement, that thirty year old indicator started to glow after about fifteen seconds, and within thirty seconds it started great. Now before hand, I had checked the plugs to see if they were working and by all indications, they were, but apparently still couldn't produce the amount of BTU's required or current flow to make the indicator appear to function.
Whereas before it would sometimes take over a minute to heat up enough to start, now only about thirty seconds (which I still think is a little long).
In any case, you may just need to replaced the plugs and save yourself the headache of getting into that dash. And glow plugs are rather inexpensive.
That's my 2¢
Oh, never did replace the old indicator.
Ron