Yeah, The EF/EL Premium Systems use an Amplifier for the Back speakers. (NO Speaker Outputs on the deck, Only 3 RCA Outs for the Back, which are connected to the amp)
The Plug on the Back of the Deck has the wires connected to it, but they are not physically connected inside.
As for the speaker Voltages, If you use an AC Multimeter, set on a low level, you should be able to get a reading on both the Positive, and Negative speaker Lines.
Whats the problem with the deck, No sound, but switches On ???