I have a sony head unit installed in my WRX. its a 4x50w, standard front speakers and 200watt rear speakers.

I can only turn it up half way before it cuts out. seems like its not getting enough power. Any ideas cause no nothing about electric's

MMMM, sounds like your speakers are the wrong impedance for the H/U. Check the wiring to your speakers for any loose connections,also check the power connections, is the head unit getting hot? Have you got bass boost,loudness switched on all the time or is the bass at max setting?

Does the head unit's display stay on when this problem happens?

Sounds like the amp is going into 'protection' mode, in other words shutting down due to poor connections or bad speakers.

Bad earth mate,i had that problem a few years ago,it played fine at low volumes but crank it up and it just cuts out.Check your black earth wire on the head unit is connected up properly :)

Both those are likely: either speakers are the wrong impedance, or shorting out somewhere against the chassis, or the earth to the unit isn't capable of carrying the current (remember that current is equal in all points of a circuit - no point in having mega power cable if the earth is bell wire ;) )

Get a multimeter out and check the impedance across speakers at the ISO connector end. You should find around 4 ohms nominal. If it's less, that's the problem.

The head unit cuts out completely, like the power been taken out of it. Why would the speakers be the wrong impedance? don't know what this means.

the car is an import (MY93 wrx) and the radio used to be alot worse (ie not loud) but i looked at it and found the power cable had some sort of japanise device installed. i took this small black square box out and reconnected the radio which allowed it to go half way. its only recently i've started wondering why it won't go any louder without cutting out, maining because the full decat has mad the exhaust louder meaning the radio needs to go louder.

The speakers won't likely be the wrong impedance, but if the wiring to them is shorting out either positive to negative, or against the car chassis, then the head unit will be shutting down to protect itself.

I'd check the wiring, then get the multimeter out and check the impedance (resistance) seen by the head unit, then look suspiciously at the power supply and earth wiring to the unit.