Adding a Defroster 
[miata.net]
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First of all you will need the following hardware:
a) A Rear Window Defroster Relay (Mazda code 67-7403). 
   The connector is located underneath the carpet, next to the left rear light 
   (when you are standing behind the car)
b) A fuse box (Mazda code 66-730). The connector is present between the antenna and the battery. 
   This box contains two 10 Amp fuses. 1 For the power antenna (if present) and 1 for the defroster.
   
The wiring for your 92 Miata is prepared for the relay and fuse box. 
You can buy these items locally from your Mazda dealer. Make sure that you secure the relay to 
the bodywork by means of a screw to earth it.

When these items are in place you have 2 alternatives.
a) the original way
b) by means of a switch.

a) The original way is to purchase the .Rear Window Defroster Timer Unit. (Mazda code 67-7TO). 
  This unit can be put in the connector, which is located underneath the dash, near the steering column. FYI this connector holds 5 wires, being:
- blue
- blue/black
- blue/red
- black
- blue/yellow

The defroster will be activated for approx. 7 minutes when you put your blower switch from 0 to 1.
The orange indicator light on the gauge panel will illuminate when activated.

b) The alternative is to activate the defroster by means of a switch. You can buy this switch e.g. 
   from MX5Parts. (www.mx5parts.co.uk/product_info.php/products_id/173). This is a simple ON/OFF 
   switch and illuminates when the lights are switched on. The installation is as follows.

1) Remove the + cable from the battery for safety purposes.
2) First you locate the above-mentioned connector with the five wires. Then separate the connector 
   from the wires.
3) Remove an empty switch cover out of the dash, o the left side of the steering column.
4) Extend the BLACK and BLUE/BLACK wires. Run them through the hole where the new switch should 
come. These wires activate the defroster.
5) Pry out the dimmer switch, connect wires to the BLACK/RED and RED wires and lead them through 
the hole where the new switch should come. These wires are for the illumination of the switch when
the lights are switched on.
6) Connect the 4 wires to the switch with little connectors (I have no idea how they are called).
When you have the pins at the rear of the switch in front of you, the RED/BLACK and red wires go 
on the right pins (top and bottom). The BLACK and BLUE/BLACK wires go in the middle pins (top and 
bottom). Do not put the switch in the hole unless you are certain that everything works correctly.
7) Then you connect a new wire to the RED/WHITE wire from the Rear Window Defroster Relay connector 
   in the boot. Lead it all the way till underneath the dash. This wire takes care of the orange 
   illumination on the gauge panel. Connect this RED/WHITE wire to the BLUE/RED wire underneath 
   the dash (see item a). The remaining BLUE/YELLOW and BLUE wires will not be used must be 
   secured properly.
8) Reconnect the + cable to the battery.
9) Test if everything works properly.
10) Put the switch in the hole.