I was afraid you were going to ask that question! You can co-opt the BLUE wire for sure. But I can't say for sure whether you can co-opt BROWN and/or YELLOW. BROWN and YELLOW have those red-circled resistors hooked to them in the base. The way it's designed, these resistors are meant to insure the motor is OFF when the controller switch is idle. Yes, I realize the motor electronics is essentially ignoring what's going on with YELLOW and BROWN since the motor runs when it should be OFF. In fact we are turning lemons into lemonade taking advantage of the fault condition of the motor chip apparently ignoring what's happening on the YELLOW and BROWN. Hope my reasoning makes sense.
So. BLUE for sure. You can disconnect from the back of the board and from the base terminal and steal the BLUE wire.
You can try disconnecting YELLOW and/or BROWN from the back of the board and see if the motor still runs as soon as you apply power. If so, then disconnect it from the base side and co-opt one or both.
As for where to get a DPDT momentary switch. The first part that came up on a Google search was this on Amazon. Somewhat spendy but shows that you might indeed find such at a local auto parts store. Note this style mounts in a square/rectangular cut-out which can be a nuisance depending on what tools you have....
