A 250sx will only start if the wire to the neutral switch is grounded (which makes the neutral light go on.
The rubber boot you are talking about just covers the wires. Take the metal cover off (if you still have one) and then pull back the rubber boot. There are two wires, one for the neutral switch, and one for the reverse switch. make sure they are securly connected.
If you want to "jumper" it so It will always start, ground the wire that goes to the neutral switch. You neutral light will always be on.
ATCnut
82 185s - sold
83 200x (hopped up)
84 200x (stock)
84 70 (lost this one to the DMV)
85 70 (soon to have lights, a foot brake, and CDI ignition)
84 200es (parts trike) - sold
85 350X (the best)
85 350X (siezed motor, dream of a poweroll kit someday)
85 250sx (frame was trash, got parted out)
86 250sx (the tractor)
86 250sx (a nicer tractor)
86 250sx (an even nicer tractor)
87 200x (parts trike, its really rough, a dog was chewing on it for a while)
87 250es Big Red (the tank!!)