Mosfet installation help

[RKG]

Hello all. I've bought myself a picoSSR mosfet to give me a better trigger response but I'm a bit confused on how to install it correctly. The gun is an asg sten and the electrical arrangement is different to most other guns. For instance the positive wire goes straight to the motor and it's the negative that goes through the trigger contacts.

I've mocked up a little diagram to show you how it's layer out. uploadfromtaptalk1440619674398.jpg

How would I connect my mosfet in this system?