Stereo power fault

Hi all.
Fitting an auxiliary fusebox system into a friends L200 today (yes, I know its not a Land Rover, but all the Mitsubishi forums have failed me and think it is a generic electrics fault.)
We needed a switched ignition feed to trigger a relay and we tried to gain this from the stereo wiring loom. We located the ignition, split it, took our separate feed off and reconnected it all (we have checked this and the connections are all good).
However now the stereo does not turn on! After some investigating with a multimeter we have found out some bizarre things...
-Both the constant 12V and switched 12V only give out 6V!
-They also only 'appear' to produce voltage on the multimeter for a second when the ignition is switched on and off. Then the voltage reading disappears.
No fuses have blown.
Any advice or points in the right direction would be much appreciated. Running out of time and patients with it so would appreciate some Land Rover magic!
Thanks
Jackattheback and a frustrated L200 Owner

Without being silly, you've checked the battery is okay I take it?
Is the feed to the radio getting warm/hot?
And does everything else work fine, lights etc, or are other things effected?
Is the 6v with the engine running or not?

we did all checks, ran the engine, checked battery, check everything else. and no luck.
But then suddenly it came back to life after leaving it over night. Very strange and frustrating and still no clue what was wrong.
Thanks for the tips though