[baumbr]

With the original Factory radio the security system passes a trickle of power through the radio. Could be a high resistance short in there. If its an aftermarket radio the installer may have shorted the security wires to keep the security codes from setting and killing the ignition. Without the radio in the circuit you'll pull more amps than just a trickle. Either condition could be why the milliamps go to zero with the fuse pulled. The other, to me, obvious place to start looking is the "keep the radio playing until I open the door" feature. Perhaps the associated door switches and relays are gunked up and passing current but not enough to run the radio, prevent it from fully turning off, etc. Did you test with the radio turned on or off?

Good luck.