Which brake light? There are 2. One indicates front pad wear, the other (which is red) indicated hydraulic problems.

I would change the trans fluid and filter if it looked funny.
I haven't done it on my 300D (was done right before I bought it) but on my SE it was easy.

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E