Why not just the two switches and the 12v supply?
Yes, even one three-mode switch with enough contact groups and proper amps.
In the case of just two switches, the TOD controller can be fried if
+12V will be connected on the TOD output (50/50 mode).
So, we decided to use a relay to block any “bad” situation for TOD controller.

I made a mistake on the
Diagram

1. Use 10 AMP fuse instead of 30 AMP.
2. The "hot" wires (+12V) of all switches need to be connected to the +12 V source controlled by ignition switch.
Sorry about that.