Maybe I missed it, but if the Merc has A/C I’d guess at water from the evaporator.
The other possibility is a leaky windshield or vent and water is trapped above or near the firewall.
By the way, I think brake fluid has a plenty strong taste.
On this model, the oil pressure gauge actually has an oil line connected to the instrument cluster. If that line has begun to leak it may be pooling in the panel above her feet and sloshing around during hilly driving and when she hits the brakes. It’s a fairly straight forward repair: Just tighten the line at the rear of the gauge or replace the line itself.
Jeff
That is some wicked old-schoolness from the oil gauge. I was thinking AC condensation as well.