Apply pressure to the pad so that it adheres to the foam.

The pad is folded below the foam.

Folded below the foam on the other side of the seat back.

This is the spring which is attached to the seat cover wire. The use of the wire is to keep the seat cover flush to the seat.

The spring must be pulled using a set of pliers and then attached to the wire.

The seat cover is now correctly attached and it can be wrapped around the back.
This way, the seat foam, the heated pad and the seat cover are all firmly attached and flush to each other.

This is how the seat looks from below. The wiring will be arranged in a later stage of the project.

Let's now tackle the wiring part of the project. This is the wire while will run from the fusebox; the color is white on purpose based on the official wiring diagram for Alfa 33 907 Series.

This is the intermediate male connector found behind the center air vents/radio. The wiring from the fusebox will reach this connector.

This is the fusebox wiring plug where the white wire (1.5 mm thickness) picks up power. Based on the electric wiring diagram the rear fog lights and the heated seats share the same relay/fuse.

The heated seats wire (white color) is soldered to the existing wire and both kept together with the help of heat shrinking tube.

Route the wire to the existing loom behind the instrument cluster.

Slide the spade connector to the intermediate wiring plug behind the air vents/radio.

A second piece of white wire (1.5 mm thickness) is needed to run from the intermediate connector to the seats. 
This new wire is actually a twin wire as we need to power two seats.

You can ease installation by using a steel wire to help route the cable inside the center console.

Insert the white cable inside the split canal with the rest of the wiring loom.

This is the intermediate female connector behind the air vents/radio. Note the twin white wires which power the heated seats.

Here are the power cables which will be connected to the heated switches.

Earth cables will also be needed. Twin earth cables as the positive ones (one pair per seat)

The earth wires are all connected to the designated threaded post in front of the gear lever on the transmission tunnel.

The earth cables along with the positive ones.

Based on the wiring diagram the switches should be wired per photo. The positive and negative cables pair should have a connector for easy removal of the handbrake lever box.

Pass the seat wiring under the carpet. The carpet has provision just below the seat where a small opening exists for the seats wiring. 
It is wise to wrap the wiring with cloth tape or PVC flexible tube to avoid direct contact of the wiring with the floorboard.

Once the seat wiring passing under the carpet has been connected to the main wiring, test the presence of 12V on the terminal connector.