This sucks!

Well I just got my Alpine deck installed today in my 68 Nova and when the guy was done installing it he said that their wasnt enough power getting to the deck, so he ran a wire from the battery to the deck to give it enough juice, which worked fine( charged me more, those damn....), but I was driving it and when I turn on my turn signals the deck turns off. As soon as I turn my turn singals off the deck turns back on. I do have a tachometer and a 3 Gauge Cluster in my car, which I know eats up volts. I do have a volt meter and i was wondering why the turn singals take up so many volts? Plus what can I do to fix this problem? New Altenator or Capacitor for my Deck or 2nd Battery?

-thanks for any advice
-Peace