You're good on your wireing diagram, but you will need a diode between the relay and your OEM trans switch, and another diode between the relay and your dash mounted switch.
This keeps the 12V from going unintended places when either switch turns on the relay.
In theory it would work without diodes, but in practice it will likely cause some sensor somewhere along the line to freak out.
12Ga is good for all your relay wires. Hell, I'd use 18 most likely, you're talking about less than an amp to switch a relay.
8Ga wire in a chassis situation is good to about 70 amps for the length of the vehicle. It might be overkill. How much current do your ballasts draw? 14Ga is good for 30, 12 is good for 40, 10 is good for about 55.
I do most of my chassis wiring with 12, with a 30A fuse there's practically no chance you'll overload a 12Ga wire, unless it's running 100ft.
This keeps the 12V from going unintended places when either switch turns on the relay.
In theory it would work without diodes, but in practice it will likely cause some sensor somewhere along the line to freak out.
12Ga is good for all your relay wires. Hell, I'd use 18 most likely, you're talking about less than an amp to switch a relay.
8Ga wire in a chassis situation is good to about 70 amps for the length of the vehicle. It might be overkill. How much current do your ballasts draw? 14Ga is good for 30, 12 is good for 40, 10 is good for about 55.
I do most of my chassis wiring with 12, with a 30A fuse there's practically no chance you'll overload a 12Ga wire, unless it's running 100ft.