I just bought a 2007 Pioneer Premier DEH-p400UB Cd receiver from a friend of mine and when I went to install it today in my 1988 Buick Regal, I got everything all hooked up and the stereo worked fine well I then turned it off and finished wrapping everything up then went to turn it back on and the stereo now will not turn on. I did a voltage test on my power wire for the stereo and am getting a reading of 11.98 Volts Can anybody help me with this??

double check all connections and fuses for that, if you don't find an issue i suggest bringing it to a place that does deck installs to have them check it out for you