Dim dome light

My alarm has a negative input for the door trigger and a negative 500mA output for dome light supervision.  I have hooked both of these up to the door trigger wire (-).  Now, when I unlock the door/disarm the alarm, the dome light lights dimly (like it is not getting enough current) and the alarm sounds.  I believe that the negative door trigger input on the alarm may be drawing all the current directly from the dome light output from the alarm.  Is this the case?  If so, should i try to block this current with a diode or something.  Everything I've read says you just hook both wires directly to the door trigger wire.  Any suggestions would help.

Thanks,

Scott