Quite possibly the resistor on the back of the gauge cluster on a
65/66... could be the same thing since all your gauges have stopped
working.
--

Spike
1965 Ford Mustang Fastback 2+2, Vintage Burgundy
w/Black Std Interior, A Code 289 C4 Trac-Lok;
Vintage 40 16" rims w/225/50ZR16 KDWS BF Goodrich
gForce Radial T/As, Cobra drop; surround sound
audio-video...
See my ride at....
Feb 2004- http://207.36.208.198/albums/86810/003_May_21_3004.jpg
Feb 2004- http://207.36.208.198/albums/86810/005_May_21_2004.jpg
Jul 2005- http://207.36.208.198/albums/86810/davescar_7_11_05_002.jpg
Jul 2005- http://207.36.208.198/albums/86810/Engine_rebuild_006.jpg

Assuming you haven't made changes or have been working in the area, the
first thing I would do is to replace the constant voltage regulator at the
back of the instrument panel.

The commonality with these three gauges is the instrument 5 volt voltage
regulator which is located on the back of the instrument panel.

Good tech article on the subject:
http://www.mustangandfords.com/techarticles/75259/

Hi,   The problem is that the cardboard "holders" on the back of the
gage, have aged and weakened,  allowing one of the gages to move.  Then
one of the elect term posts touches the housing, it grounds and shorts
it out.  FIX:  Remove the gage cluster. Carefully remove the CB board
and look at the gages, see two posts on each?  At each side of the gage
cardboard, there is a little tab that locks it to the housing, to hold
it.     What I did, was made a tracing of the cardboard and made a
duplicate out of approx 1/16 inch plastic.  I actually used a piece of
plastic from when I added the leaf gutter guard to my house.  It is
hard but easy to work with and it does the job just fine.   you cut the
tracing and copy it to the plastic using a pencil.  Then use scissors
and razor blade to cut the plastic to the shape.  Use a drill to cut
the holes for the elect posts.  Then check the fit.  Be sure to make it
accurate and include the two tabs on each side of the gage, that locks
it into the housing.  THIS IS what moves and allows posts to contact
the housing.   It sounds hard,  but it is easy.  I did the same on my
69 mustang, me and an engineer at honda figured it out

This is the fix !   Do NOT buy a new circuit board or CVR until you try
this.   Stan