IF … I am recalling correctly from my old electronics days (that was 50 years ago … if not, I will be corrected ).
Based on the drawing Aristides provided (see my corrections for the L and R indicator mix up), I also added the circuit path from IGN (+12V) to ground (-12V), the following is the wiring:
Upper left corner, from IGN switch, through NC contact on hazard switch, +12V goes to flasher terminal 49, which apparently is the +12V feed to the turn signal circuit in the flasher, and feeds the turn signal switch through terminal 49a.
The turn signal switch feeds +12V to the outside lamps, which have their other side connected to ground to complete their circuits.
IGN +12V goes to the indicator lights in the tach and the speedo, with the other side of the indicator lights going through diode and connecting to the +12V from the turn signal switch to the outside turn lamps. The diodes only allow current flow in one direction, so the indicator lamps are only ON when the outside lamps are OFF. The diodes don’t allow current flow through the indicator lamps when the flasher IGN +12V is OFF to the indicator lights (+12V is ON through the turn signal switch to the outside lamps).
If the diodes are bad (allow current to flow in both directions, then the indicator lamps will light under both conditions (when IGN +12V is to the indicator lights and when +12V from the turn signal switch is to the outside lamps).
First, use an ohm meter to check the diodes with the meter’s (typical) 1.5V battery. You should read continuity through the diodes in one direction, but not when you switch the meter leads to test with the polarity reversed.
IF that is good, then do this check at 12V (diodes might be good at 1.5V, but shorting out at 12V): put +12V to one side of the diode and -12V to the other contact of the light, then switch the polarity of the 12V.
The indicator lights should be ON when +12V is applied to the side of the indicator light not connected to the diode, but should NOT be ON when the +12V is connected to the diode connected to the light and -12V connected to the other side of the light.