Alright, the diode is not needed to achieve either both pumps on in manual or just the auto pump on by itself.
The auto pump has three wires, brown, black and brown/white.
-black is ground
-brown is auto
-brown/white is manual operation
Connect the brown wire on the manual pump and the brown/white wire from the auto pump to one side of the 3 position switch.
Then you will need to add a third wire to your harness from the ebox to the auto pump and use it to connect the brown wire on the auto pump to the other side of the three position switch.
Position 1 = both pumps on in manual
Center = both pumps off
Position 2 = one pump off, one pump running in auto.