I have payed much attention to this. As I see it, the current draw form the aux fans causes this I believe. I am inclined to think this as some folks have had their fans and wiring changed after repeated fuse #44 failures because of too much current draw. And I have seen the drop in lamp output just as the fans kick in.

my $0.02

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'02 C240
'00 LR Discovery
'72 280SE
'67 280SL

---past---

'79 280E sold (RIP)
'86 300E sold
'87 300SDL sold
'90 Laforza sold
'95 320SE sold
'98 ML 320 (sold)