the problem was...
my cd changer. it had never worked since i bought the car last december. i didn't notice this problem until recently when i got a 2nd car and was not driving the 500e daily. there was actually a cd jammed in the player that was somehow making the changer try to do something, hence the issue. as soon as i took the cd out, problem was gone...and my changer works just fine.
i've heard about drainage issues related to cd changers and also power antennas. if you have a changer, try unplugging the power to it...or just pull out the fuse in the back...and see if the reading drops.
my first multimeter was a $20 job from home depot. i really didn't know what the heck i was doing and got one that wouldn't read high enough. a friend of mine brought his over, much better one, and we had no problems getting a reading in-line with the neg cable and post.
let us know how it turns out.