Cool stuff in that reply! Thanks a lot, I'll test it out tonight!

Does it dim whenever you let the RPMs drop, even if you're not brakeing?

Only when I break, switch on the fan, blinker, lights, etc.

If no - look at ground points. The current through/from the brake light might be raising the voltage at a [bad/corroded] ground point so that things get confused (charging regulator etc)

Any ground points in the involved circuits or only those close to the alternator?

Any similar effect if you turn on/off other heavy consumers like fan or rear de-mister?

Yes! The amount of the drop varies, but I get it basically in all electrical consumers. With the blinker for example the effect is so small, I can only see it if the rpms are at idle. I really think it's because the battery is not full, yet. I'll check all that again tonight. It read 12V when I left the car last night. I'll see what happens now.
The battery is a Westco dry battery, only a few months old. I'm not quite sure if I should trickle-charge it or not. I usually tend to leave it to the alternator and only charge it with a charger if it is absolutely necessary.

increase RPMs to about 2000, voltage across battery should rise to at least 13.8V, but not over 14.4V.

It shows just under 14V on the highway (3-4k rpm).
If the problem persists even with a full battery, I'll have it checked out.
Thanks again for your help!