I used relays cause I didn't want to wait on a box - and - the relays were selected to be 10mA of draw

- easy as, minimal drain.
The transistors would add load to the circuit just around 1/10th of the relay - all insignificant though..
I got the relay box together the same day (saturday).
Why the relay box or the other box? Simple - they don't want the globes of the trailer (500mA + ) of load on the existing circuit - hence why the relay box is just as valid as the transistors.