Obtain the following materials

  1. Double sided adhesive backed sheet material
  2. 70 ohm/foot Nichrome wire
  3. adhesive backed copper foil ( 1/4" x 1/4") 
  4. 2 conductor stranded wire (28AWG ) about 1 foot

The double sided adhesive backed paper acts as a holder for the coil. 

The design generates about 1.5W of heat, at 15VDC (your 12VDC system actually varies from about 19V to 11V)

Measure enough nichrome wire to give about 110 Ohms of resistance, you can do this with a multi-meter or just measure the wire ~ 18"

 Strip about 1/2 of insulation off your lead wires 

Twist the lead wires with the ends of the nichrome wires

Put the copper foil on a flat surface and solder the twisted end to the copper foil.

Remove the copper foil and place them on the paper ( make sure you pull the adhesive liner away so that the sticky part is exposed).

carefully route the nichrome wire into a coil moving back and forth (see my example).

Once you are done, completely remove the adhesive liner and fold the remainder of the sticky paper over to insulate the wires.

Solder the other side of the lead wires to the ground and V+ battery points.