Tacho / brake light problem

[Guy Weller <GuyWeller@compuserve.com>]

Hi Frank,
This is surely an electrical problem.
Is it only the brake lights that cause the problem ?
What about when you switch on headlights / main beam ?
If it is when any other high - current  demand item is on then I would
check the  tachometer
connections, particularly the earth (ground). Check also the earthing
between engine and body.
If it is only when the brakes are on then try checking out the earth to the
brake light units.  If this is poor you wil get a whole Royal Family of
"Prince of Darkness" effects !!

Guy