I am planning to use my cigarette lighter (15 AMP fuse and 12V) as a source to split to create 2 sources of power (12V) to run 2 O2 simulator (passenger and driver side) which use very little power as it output 0.0/0.7V every 3 seconds to the PCM/ECU.

The length for each wire should not be more than 3-4 feet for
each side.

According to the wiring chart.. I should use 14 gauge wire... but I think these are too large... can I safely and reliability use 18 gauge wire?

 

Personnally, i don't think you will have problem since i've done it many times with leds strips and and lighting devices on much smaller gauge wires: 22-26awg.

You can use an inline fuse for peace of mind, but since the lenght of wires are so short, i really don't think it should be a problem.

According to most chart, a 18g should handle well the amperage of your devices with that lenght. I use this:http://www.rbeelectronics.com/wtable.htm

You should be ok with 18awg for a 12v, 15amp circuit up to 10 feet.