For a front wired M4, the wiring is typically like so:
1. Negative (black/-ve) wire. One end is in the battery connector in the handguard...the other runs under the outer barrel around the hopup unit, into the front of the mechbox just above the safety arm, past the trigger, down through the slot, past the hole where the motor would enter and out through the bottom (rear hole). Tuck the wire in tight against the mechbox shell, and as low as possible around the bevel gear area.
2. Positive (red/+ve) wire. One end is in the battery connector in the handguard...the other runs beside the back until it get's into the mechbox. Then it is typically attached to the upper switch contact.
3. Positive (red/+ve) wire. One end is conntected to the lower switch contact, runs along the slot (on top of the -ve wire), and out through the bottom of the mechbox (forward hole).
If I recall correctly, the red wire is 6.5" long from the bottom of the mechbox to its tip and the red is 5.5" long (might be 4.5"...but I think it's 5.5"). If you are running new wire and want to be really sure...mount the grip before you cut the wires to length, route the wires, install the motor and cut the wires to length.
Or....you can just google this yourself next time...took about 10sec to find this picture (it's from DonP's website)
