Battery slowly drained

Is this your main battery or an auxilliary? If aux, why not disconnect everything at the positive terminal except for the main cable connecting it to your relay/black box etc. See if it charges off the main battery/alternator after a drive and then check again a few hours later. If it has dropped again, I'd say the battery is faulty or the "black box etc" is stuffed.
If you don't have dual batteries, then there is a chance the battery is stuffed too, or you have a slow drain from another source. But if you've taken all the fuses out etc, there should be nothing there to drain it.
Really we need more info to try to assist you.

"Any ideas of what I should be looking at?"

an autoelect..
your mate may have bolloxed up a wire while fitting radio. Cause its only happened since radio went in?

Have you got a multimeter that reads Amps?
If so, set it to 10A DC or bigger range and connect it between the + battery terminal and the wires that went to this terminal. Put back all your fuses.
DO NOT TRY TO START THE CAR WHILE THE METER IS CONNECTED!
Now read what kind of current drain you have on the meter. Then try to disconnect things, one at a time, and see when the current drain stops.
Voltage readings can be rather timewasting in that situation - you might have a battery that's on its way out.
The only things usually permanently connected to the battery is the alarm, the radio's memory power input and the clock - everything else should be off with the key out of the ignition.
If yours is a modern car with a computer engine management system, please disregard the above advise.
Klaus

what sort of car? I had that happen in the past and it turned out to be the boot light was staying on as the tab with the switch had bent slightly. Could be the glovebox light also!

Hi Rob, Ussually the radios have a power supply that connects to constant power to keep the presets and time corect and another that connects to a switched power supply (acc. or ign.) that powers the radio when you turn on the ignition, maybe they are wired wrong.
Another thought is, has it a separate amp? if so is it switched to go off with the ignition?
Will

Why not try disconnecting the wiring from the battery complete and checking the battery outside of the car first. It probably wont be a bad idea to take the battery to a battery dealer and ask them to check the battery.

Other than that I had the same problem with my old 82 toyota hiace. Turned out that my in car phone kit was the problem. the cradle had a bit of electronics in it which was causing my battery to flatten after about five hours.
It didn't help that the battery was on it's last legs as well off course.

Sparkie(IE not Y) ;-)

Remember the alternator has a direct connection to the battery.Unhook the output carefully and check with the ampmeter or connect a 12v bulb (5w will do) between the unhooked alternator lead and the bare output terminal of the alternator.There should be no current, if there is the diode(s) are shorted.