I have a 2005 pathfinder and want to piggy back the main/full beam to power a light bar. I do not want a dash switch What colour is the full/main beam wire that comes from the lights Jon

one wire will for high beam
one wire will be for low beam
the last wire will be ground wire
as fog light have only one beam you can use the low beam wire and parallel wire it to the fog lights
that way you have the head lights still and the fog lights as accessories correctly wired according to regulations
how to do it simply-- find the low beam power wire ( light switch on low beam and power at the light bulb check ed with a multi meter set on volts
solder , piggy back, or double plug in a connection into that wire and run one wire to the left fog light and one to the right fog light connect each fog light to an individual ground wire and connect those wires back to the ground wire for the main lights
Now having done that simple addition to the lighting system another problem has come up
The wiring will now be too light to carry fog light current and or the light switch will burn out
To over come this problem, the circuit needs a relay
fit a relay on the bike and now run the low beam wire to a terminal of the relay
run the other terminal wire to ground
this enables the relay to operate when the low beam is switched on but at this will not switch on the lights yet
now you have to run a heavy gauge wire (fused at the battery) to one of the power terminals of the relay and from the other power terminal to the connection for the fog and low beam lights
Now it all works switch on the high beam and no for lights
change to low beam and the fog lights come on as well
All nice and legal
Of course if you don't care about being legal just buy a 30 amp toggle switch and run a fused wire directly to the switch from the battery to the lights and then to ground from the lights
You will know that the generator / alternator for the bike will not be able to run the fog lights with or without the main lights so what will you have achieved

you will have to fit a relay into the system. Tap a wire into the high beam power wire for the headlights. (Piggy back connection) take this wire down to the fitted relay to a coil pin on the relay. connect a wire to the other coil pin of the relay and return to the dash area to an control switch through it then to an earth point.. Run a wire to an inline fuse directly from the battery ( fuse close to the battery as it is a high current line) down to one pin of the relay. connect up the power wire for the light to the opposite pin on the relay and connect the earth wire of the light to an earth point

According to road regulations you cannot have spot lights on by them selves and they must only operate in conjunction with high beam operation.. The setup as I describe allows you control of when the spotlights will be on ( switch in the dash) and if on will be extinguished when low beam is selected as per regulations. The relay ensures that the trigger wires do not carry full light amperage so light gauge wires can be used and only the power wire from the battery to the relay to the light need to be heavier gauge.

My first guess would be the headlight dimmer switch,,but check the lighting module to be sure

Well the dash lights and headlights are on two different circuits, but both are controlled by the headlight switch.
Do you have power in and out of the switch for both circuits ?

if you hook the driving lights to the low beam circuit you will be in breach of traffic regulations the regulation reads that driving lights can only be operated with high beams and on selection of low beam , the lights have to go out
added to that is the clause --the driving lights most be extinguishable when on high beam by the use of a separate isolation switch
now the circuit is really simple
you place a relay for each driving light in a suitable place
you run a fused wire from the battery to the relay and then to each light
to make those relays work you connect a wire from the existing high beam relays connections(m piggy back or like connection) so that when on high beam the trigger power that puts the headlights on also triggers the driving light relays
Now to meet regulations you run ground wires for the driving lights back to a switch in the cabin then to ground

Did you check the relays? or the High beam switch? Usually you can trace the wiring down from the switch to a harness at bottom of steering column use test light there too.Use a Test Light at the headlight connection make sure that you have power Low beam then test high beam. If you can't figure out use of Test light, it has a ground clip and a probe (Sharp end)Ground the clip on something metal (on car) use the probe to touch wire (one at a time) in the socket where the headlight connections. 1 wire is ground 1 is LOW beam and other is HIGH beam. may need a mechanic to check the switch. The light on the tester will light up IF you have 12 volts. NO LIGHT NO POWER!

check the ground connection of the bulbs. check the light switch. the headlight relay is located under the dash or under the hood.

its the switch unit itself

The main/high beam relay/switch must be stuck on high presuming the high beam stalk selector does not operate. Probably needs replacement.