My jbl stereo wont turn on. when i turn my truck on it just lights up and then goes straight back off

only a quess if stereo isn't working
the JBL amp
you can retro fit another amp
How to add amplifier to your Toyota Tundra JBL keep your stock stereo...

The connections are pretty straight forward, but it will depend on what amplifier you're driving them with. Normally, I'd connect the two TR to one side of a stereo PA amp and the two MR to the other side. All four cabinets are rated at 8 ohms, so running tow parallel per side would give you a 4 ohm load on each channel which would be a common load for most stereo PA amps.
However all this would depend on your gear and what you need to accomplish. If what your question really meant is how do you connect all 4 to one channel, you can't as in most cases the total impedance would be too low and you'll fry your amp.
If you can be more specific about your system we could be more helpful.

Eliminate the power conditioner as a variable and get back to us.

http://www.jblpro.com/catalog/General/Product.aspx?PId=27&MId=5

Manual:
http://www.jblpro.com/catalog/support/getfile.aspx?doctype=3&docid=859

The fact that the LED's also go on/off points to a power supply fault.
First check the mains power plug for the power-pack, as if it's supplied by 2-pin 2 amp type, the adapter contacts could be loose.
If not this, or directly into speaker, check DC socket, or a cracked internal board track or dry solder-joint.

It can be bad filter caps, read the service manuals here: http://manuals.harman.com/JBL/HOM/Service%20Manual/PB10%20sm.pdf
http://manuals.harman.com/JBL/HOM/Service%20Manual/PB12%20sm.pdf

Or bad power amp module, take the voltage reading per SM, I mod my, http://s807.photobucket.com/albums/yy352/budm/JBL%20PB-12%20mod/

unfortunately it probably burnt up. If all the wiring's good like you said it is it's probably just fried... My advice is send it back if it's still under warranty, they'll either repair it or get you a new one. If its not under warranty you can pay to have it fixed or just buy a new amp. I you buy a new one go Kenwood, or Alpine, i've had problems with jbl in the past...

check you speakers and power source make sure all connection is correct

remove seats and pull carpet. now see which wires they melted with that torch. Tape them up so they aren't shorting out any more as best you can.

Mine has a blown power supply transformer. There are 2 fuses on the power input board. One of the fuses on mine blows instantly because the primary on the tranformer has gone short cct. A $200 part from Jands !!!

Well...lets start with the obvious. Check fuses and NEVER piggyback any stereo or security stuff into their fuses...you dont want the lights to shut down during a night drive!!
Check under the hood for a relay center...also has some fuses there as well.

The Daytime Running Lamps are a "straight through wiring configuration". Doubt it even goes through a headlight switch at all.
But the regular lights do.
Unless you've worked on the front of the car and forgot to plug the headlights back in, do this...
Turn the light switch on...looking at the instrument cluster , can you make the high-beam-indicator light come on?
As you rotate the headlight switch knob...does the instrument panel lights brighten or dim? You probably have a faulty headlight switch...auto stores have them...they simply plug in.