Yes, that is very interesting. It sounds to me like you see a direct relation between the modulated brightness and the speedometer -- maybe the pulsing follows the odometer steps, which you can perceive if you watch the number dial closely instead of the road (not a suggestion!).

Just to confirm this suspicion, before attempting to guess at the reason for the interaction, why not try disconnecting the vehicle speed sensor to see if it stops doing this? Be aware, the ECU will set a 311 and it will affect your idle if you leave it disconnected, but sounds like it is worth a try.

The best place, I think, is in the junction box in the rear tire well (wagon) or trunk. Check this link for a clue: http://cleanflametrap.com/speedoDiag.html
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Art Benstein near Baltimore