Carputer problem

Hi guys im new here so sorry if iv posted in the wrong section.

Basicaly my problem is this,
iv just built a carputer and my car psu came today.....wired it all up and plugged it in to turn it on a install windows but when it turned on the led on the motherboard stays on but the led on the psu keeps flashing on and off like its not getting a constant power. The motherboard fan comes on and goes off with this flashing led aswel. So its almost like its turning on and off within a couple of seconds and i am stumped as to what to do next.

So any help would be great.
Cheers guys

Any one with any ideas?
Pulling my hair out here guys =(

What is the manufacturer of the PS?
is it new?
What is the rating for the PS?

Check the polarity of the lead that goes to the on/off on the mobo
Check if you have hooked ALL the ATX power connectors from the PS to the Mobo
Check manual for trouble codes
Describe your devices, how much watts/amps it the system drawing?
Consider calling the manufacturer of the PS if possible

SUGGESTION:
Use a regular PSU in the house (or apartment/flat/workplace) to do all your installing.

As above. But also sounds like it could be the on/off button lead. if you now have it hooked up to the psu. Disconnect it and reconnect to the mobo

Could be the CPU, or Memory not seated correctly?

I Always suggest PC install and config on a bench (in house) with a known good p0wer supply and d1splay. Make sure all the media is copied and the player of of your choice can play it, test the s0und and d1splay set up.... then install in the car. This leaves install items and car p0wer supply to deal with in the garage.

Good luck