[tsubaki]

I probably won't be able to tell you how your meter is supposed to work but normally if you reverse the polarity it will show up as a negetive number on the meter.
Example in a 12 volt system: if the black meter wire connected to a positive wire and the red meter wire connected to the negetive wire it will read -12 volts. Reversing the leads, it will read 12 volts, therefore the red lead is connected to the positive wire and the black lead is connected to the negetive wire.

Now,.....depending on what the "mechanic" that last serviced this spiderweb, you could end up with a number of different possibilities.
Initally it sounds like they are switching the ground. One way to find out is to make a jumper from the battery negetive terminal and test between it and the lights. If you get a reading that is what they have done.
Normally "most" people will observe the black as a ground, that ain't to say this is the case. Gotta see.