low voltage in harness

i have 2001 dakota and i was instaling a pre amp and the power wire got pinched somehow and then my stereo wouldn't turn on. I bought a new stereo and wired it up but when i turned it on the there was no power so i checked all the wires and i'm not getting 12 volts out of the power wire from the factory harness its more like 8-9 volts of power. any ideas on how to fix this or what may be the problem? and do you think my first stereo is still good or is it fryed?

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2001 dakota r/t 1 15"RF HX2
1 1000BD power amp 1 1 farad cap 4 channel amp

Your old stereo could still be good it just wouldn't work with that low of voltage.

First, what did you measure that gave you a voltage of 8-9 volts? Was it just across the constant wire and the ground wire? If so you may want to try measuring the voltage from your constant wire to the negative terminal on the battery. If that reads 12 volts you may have a bad ground. If it still read 8-9 volts then you may need to check/replace your constant wire. You should also check the voltage of the ignition wire to see what you get.

If you are sure that you pinched a power wire during the installation, you have a blown fuse.  Which wire shows 8 to 9 volts?  The constant or the switched wire?  There are 2 fuses that supply power to your radio.  Only one of them is labeled radio on the fuse chart.