95 civic and sony header, need help!

Replace the 7.5-amp "BACKUP" fuse in the underhood fuse box.

7.5 backup fuse?  do you mean the one for the radio and the cigarette lighter? that's a 15 in slot 23 and that was fine. i checked all the 7.5's i could find and they were all okay.   well i did manage to get the hu working, i hooked up the hu side's red and yellow to the accessory 12V and it worked, it even kept time.  is it alright to keep it like that? or should i diagnose fully what's causing the constant to put out only 4.5?

your car may have two fuse boxes...one under the hood and one in your car somewhere.   if replacing that fuse still doesnt work....then your constant wire has a short in it somewhere.   if that is the case then cut it, cap it off and electrical tape it up, then run a fresh new constant wire from the battery and install probably a 10amp fuse.