Cadillac Repair: 1986 Cadillac Deville, fuse box, fuel gauge

Question
My son attempted to install a new head unit to replace existing radio.  He cut the orange power wire from the original harness.  He repaired the wire and re-installed the original radio and it works just fine.  However, his fuel gauge stopped working along with his air conditioning.  We checked the fuses in the boxes by the steering column and on the passenger side by the air conditioning vent and all are OK.  My auto book says that there is a 3rd fuse box but we can't locate it.  Any suggestions on how to solve this problem or on how to locate the 3rd fuse box.  Thanks!

Answer
Hi,Paul

When the radio was installed,what ckt did you use for the power,try removing the radio and see if the fuel gauge returns,the 3rd fuse box is a relay center located under the right side of dash.let me know on what wire was used for power to the radio,also use a voltmeter and check the voltage to the wire that was used without the radio conected,if the wire shows 16 volts,do not use that wire,the ac controls and fuel data use that voltage for displays.


ED