Astra-F 17DR weird electrical earthing problem :o(

Hello group.

Feels like i've fried something trying to connect amplifier in place of
standard radio unit (philips). I've noticed pretty strange behavior of the
electrics driving home that night but took no action. There is a
unidentified earth fault somewhere in the system. When i connect a fuse on
position 24 (radio, courtesy lights, hazard flasher, etc) it grounds up
( but when the fuse is removed then i can start the car normally. I've
disconnected radio, dual display unit, dashboard, every relay, door's
wiring, trying to locate it but it's there still. Also for the first time
since the aquisition of the car i've noticed that the airbag's light is up
constantly. When I switching hazard lights on it doesnt flush but stays
on...very strange.
the car is astra-F estate with 17dr gm diesel (l-registration).

Confused ( please help

 

few more details:
a) if anything of the interior lighting is switched on - the short circuit
(sc) exists.
b) if lighting is off and any door is open - sc exists but goes off when the
door is closed (

thanks in advance