I had the same problem with it not turning off, but thats just because your remote turn on isnt working properly. So the volt meter just gets a constant 12 volt and doesnt turn on and off with the ignition. It should work if you have it wired into the amps turn on (how mine is), but i would try to splice the actual turn on wire from the head unit and split it to the amp and meter. See if that works.
Also you said you dont have the cap in, so does that mean your amp isnt plugged in an running? If so then the volt meter is getting power, but its not grounded so it wouldnt be working right. Here is what i did, i ran an 18 guage speaker wire to my front battery and put the start up wire into the amp. But it depends on what voltage you want to read, i just wanted to see my Voltage loss.