The surprise reveal went down Monday night in L.A. after the singer and some friends celebrated her assistant's birthday. When dinner ended, Madison handed her assistant a bag, but it turned out to be empty. It was also a ruse.

Check out the vid ... you can see Madison pull out a key fob to a brand new Mercedes-Benz C300. Suffice it to say ... Madison's assistant was stunned and damn near speechless.

Helluva gesture, especially 'cause Madison knew about her assistant's car trouble for a while. We're told her assistant's 12-year-old whip kept breaking down. It's a car she used a lot to run Madison's errands and whatnot. Madison also knew her assistant's dream was to one day own a white Benz. Madison delivered, and then some ... a white-on-white model that tends to run upwards of $50k.

Goes without saying, Madison is super close with her assistant, and knew this would blow her mind. She was dead-on, as you can see in the video -- which also reveals a sweet new nickname for boss Beer.