When taxiing to the gate a smell of electrical smoke was detected by a flight attendant. Smoke continued to develop and the plane was evacuated at the gate. A fire subsequently substantially damaged the fuselage skin and destroyed the aft interior of the plane. The investigation found the origin of the fire in two electrical wires that provided 115-volt alternating current (AC) to a utility plug in the right stowage closet, and 28-volt AC to lights in the stowage closet and the emergency drawer at the bottom of the closet, respectively. The wires (L1156AM18 and M504D16) were found to have a slack of about 10 centimeters to 15 centimeters for a distance of about 45 centimeters to 55 centimeters between supports and became pinched between the inboard-facing flange of [an intercostal] and the recirculation fan duct because of a lack of provision for clamping or other means to secure the wires.