d21 will work fine it is the same as R21 just at different points on the board. if you look under a magnafying glass you can follow the trace from D21 to R21. they are conected.
let me explain,
this mod works off of the pwm (micro pulses) from the led those pulses are wired to the trigger, the controller reads those pulses on the trigger as multi preses of the trigger. now as for the batteries, when the batteries are full, the pulsing happens so fast the controller doesnt see a pulse it just sees the voltage, and thinks you are holding the trigger. as the batteries are depleated, the pulsing slows down and the controller then reads it as many presses. so when the batteries are full, it will fire a single shot (as if you pulled and held the trigger) if this happens when you push your mod button, you have done it correctly and the batteries need to be depleted. if you press your mod button and nothing happens at all, you have wired something wrong. if you press the mod button and the controller turns off, you have most likely shorted the top trigger pin with the center, and last but not least if you press the mod button and the screen shakes and acts crazy, you have shorted the center pin to the bottom pin.
let me make this easy for you, consider it my gift to a new member, take the rumble plug, cut the wire off the rumble, cut the black short so it cant touch anything, take the red wire and solder it to one side of your button, solder the other side of your button to the center trigger, and you are done. the same battery pricable will apply, but you and your dad will not have as much heartburn trying to solder to the via or led.
good luck.
