Power Problem

I am trying to install a new head unit into a 95 honda civic.  The process is cake up until I find that the blue/white wire (which is the constant 12V wire) has 4V going through it.  It supposed to have 12V so there is the problem.  If anyone has a schematic diagram of the car or could help me troubleshoot that would be great.  I thought it might be the radio fuse, but i checked that and it's good.  I don't know if there is a seperate fuse for the constant power wire or not.  If so, I need to know where.  Thanks.

If you want to save some time on finding the problem and go around it, look for a constant wire in the dash elsewhere,   try your ac controls ect.    or go stright to the steering collum2009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR

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yes there is a separate fuse for constant pwr, and it's in the fuse panel that's in the engine bay. i'm just going on memory but i think it may be called "back-up". all problems can be resolved with a hammer

7.5 Amp fuse, as jimmeezgolfvr6 indicted it's in the fuse panel in engine bay. Right top corner of the fuse panel. Be careful whose advice you buy, but be patient with those who supply it.